Chronic political instability in Pakistan and Islamabad's military efforts against the Taliban and al-Qaeda have raised concerns about the security of the country's nuclear weapons. Some observers fear that Pakistan's strategic nuclear assets could be obtained by terrorists or used by elements in the Pakistani government. However, U.S. officials have generally expressed confidence in the security of Pakistan's nuclear weapons. The collapse or near-collapse of the Pakistani government is probably the most likely scenario in which militants or terrorists could acquire Pakistani nuclear weapons. Gary Samore, then-National Security Council Coordinator for Arms Control and Non-Proliferation, stated in an April 2011 interview that The Pakistani government takes the nuclear security threat very seriously, and they've put a lot of resources into trying to make sure that their nuclear facilities and materials and weapons are well secured. There's no lack of recognition that this is a very important issue, and there's no lack of incentive on the part of the Pakistani government to maintain control. What I worry about is that, in the context of broader tensions and problems within Pakistani society and polity—and that's obviously taking place as we look at the sectarian violence and tensions between the government and the military and so forth—I worry that, in that broader context, even the best nuclear security measures might break down. You're dealing with a country that is under tremendous stress internally and externally, and that's what makes me worry. They have good programs in place; the question is whether those good programs work in the context where these broader tensions and conflicts are present. Pakistani efforts to improve the security of its nuclear weapons have been ongoing and have included some cooperation with the United States; former Pakistani President Pervez Musharraf told a journalist in 2009 that Islamabad has "given State Department nonproliferation experts insight into the command and control of the Pakistani arsenal and its on-site safety and security procedures." Moreover, following the 2004 revelations of an extensive international nuclear proliferation network run by Pakistani nuclear official Abdul Qadeer Khan, as well as possible connections between Pakistani nuclear scientists and Al Qaeda, Islamabad has made additional efforts to improve export controls and monitor nuclear personnel. The main security challenges for Pakistan's nuclear arsenal are keeping the integrity of the command structure, ensuring physical security, and preventing illicit proliferation from insiders. Some observers are also concerned about the risk of nuclear war between India and Pakistan. The two countries most recently came to the brink of full-scale war in 1999 and 2002, and, realizing the dangers, have developed some risk reduction measures to prevent accidental nuclear war. Nevertheless, Pakistan continues to produce fissile material for weapons and appears to be augmenting its weapons production facilities as well as deploying additional delivery vehicles—steps that will enable both quantitative and qualitative improvements in Islamabad's nuclear arsenal. Some observers have also argued that Pakistan's expansion of its nuclear arsenal, development of new types of nuclear weapons, and adoption of a doctrine called "full spectrum deterrence" indicate "the growing nuclearization of Pakistan's national security policy." These developments have "sparked international concerns about the safety and security" of the country's nuclear weapons, as well as raised "concerns that these weapons might be used through intentional, inadvertent, or accidental launch in a crisis or in limited warfare with India." A 2013 State Department report explains that India and Pakistan's governments view nuclear weapons as "vital to their security," adding that these states' respective decisions to pursue nuclear weapons stem largely from their troubled bilateral relationship, assessments of threats posed by each other (and China in India 's case), perceptions of enhanced national power or status derived from possession of such weapons, and domestic politics. The nuclear programs are popular within each country and are protected by strong institutional and domestic political constituencies. In view of these factors, international pressure over a period of decades has had little direct impact on the attitudes of India and Pakistan concerning nuclear weapons. Islamabad is expanding its nuclear arsenal and developing of new types of nuclear weapons. Special Representative for Afghanistan and Pakistan Ambassador Richard Olson told the House Committee on Foreign Affairs on December 16, 2015, that the United States is "concerned" about "the pace and the scope of the Pakistan's missile program, including its pursuit of nuclear systems." The Administration also worries that "a conventional conflict in Southwest Asia could escalate to include nuclear use as well as the increased security challenges that accompany growing stockpiles," he added. Similarly, Defense Intelligence Agency Director Vincent Stewart expressed concern in February 9, 2016, testimony before the Senate Armed Services Committee that the growth of Pakistan's nuclear arsenal and Islamabad's "evolving doctrine associated with tactical nuclear weapons, increases the risk of an incident or accident." Pakistan's nuclear energy program dates back to the 1950s, but it was the loss of East Pakistan (now Bangladesh) in a war with India that probably triggered a January 1972 political decision (just one month later) to begin a secret nuclear weapons program. Observers point to India's 1974 "peaceful" nuclear explosion as the pivotal moment that gave additional urgency to the program. During the 1970s, Pakistan began programs to produce highly-enriched uranium (HEU) and plutonium for use as fissile material for nuclear weapons. HEU and plutonium are the two types of fissile material used in nuclear weapons. Pakistan first produced fissile material for its nuclear weapons using gas-centrifuge-based uranium enrichment technology, which it mastered by the mid-1980s. Gas centrifuges enrich uranium by spinning uranium hexafluoride gas at high speeds to increase the concentration of the uranium-235 isotope. Islamabad gained technology for its nuclear weapons program from many sources. This extensive assistance included uranium enrichment technology from Europe and China. Islamabad has received Chinese and European assistance for at least some of its plutonium program. A 2001 Defense Department report states that "China supplied Pakistan with nuclear material and expertise and has provided critical assistance in the production of Pakistan's nuclear facilities." This assistance included help during the 1990s with a "plutonium production reactor" and ring magnets from a Chinese entity for Pakistan's enrichment program. China may also have provided "nuclear weapons design information" to Pakistan. In addition, China and North Korea have supplied missiles to Pakistan. One 2013 non-governmental report argues that Pakistan has been procuring components for its enrichment program from foreign entities. The U.S. and other governments had information during the 1970s that Pakistan was constructing a uranium enrichment facility. A.Q. Khan has stated that Pakistan began enriching uranium in 1978 and produced HEU in 1983. Pakistan told the United States in 1984 that it would produce only low-enriched uranium (which is not used as fissile material in nuclear weapons), but a 1986 Arms Control and Disarmament Agency (ACDA) memorandum indicates that Islamabad had violated the pledge and added that Pakistan had "overcome the last major obstacle to nuclear weapons by producing enough high enriched uranium for one or more nuclear devices." Pakistan's "HEU production capacity" in 1987 was "sufficient to produce one bomb per year," according to an ACDA memorandum written the next year. The country's main enrichment facility is a centrifuge plant located at Kahuta; Pakistan may have other enrichment sites. A 1985 CIA report described a possible Pakistani plan to "build a plutonium production reactor" and Pakistan has operated the 40-50 megawatt heavy-water Khushab plutonium production reactor since 1998. Islamabad has been constructing at least three additional heavy-water reactors, which would expand considerably Pakistan's plutonium production capacity, at the same site; whether all four reactors at the site are operational is unclear, according to nongovernmental expert reports. Additionally, Pakistan has a reprocessing plant at the New Laboratories facility of the Pakistan Institute of Science and Technology (PINSTECH) and is apparently constructing other such plants. Sources identify 2000 and 2002 as the dates when the PINSTECH plant began operating. Pakistan also appears to be constructing a second reprocessing plant at PINSTECH and may be completing a reprocessing plant located at Chasma. The United States had information during the 1970s and early 1980s that Pakistan was pursuing nuclear weapons designs, but exactly when Pakistan produced a workable nuclear explosive device is unclear. According to a 1978 State Department cable, the United States estimated that it would take Pakistan "at least" three to five years to produce a nuclear explosive device. A 1982 National Intelligence Estimate (NIE) assessed that Pakistani nuclear testing would be" feasible for the first time" in late 1983 or 1984. A 1985 National Intelligence Council report states that Pakistan "probably [had] a workable design for a nuclear explosive device" and was "probably ... a year or two away from a capacity to produce enough" highly enriched uranium for such a device. According to a 1991 NIE, Pakistan had "a viable nuclear weapons design and has components that it could assemble into nuclear devices on short notice." Islamabad attained such a capability "by the end of the 1980s," said a 1993 report to Congress, apparently from the National Security Council. Pakistani officials have cited 1984-1985 as the period during which Pakistan acquired the capability to detonate a nuclear explosive device. A.Q. Khan stated in an interview published in May 1998 that Islamabad "attained" the capability to detonate such a device "at the end of 1984." Similarly, Khan reportedly stated in a January 2010 speech that Pakistan "had become a nuclear power" in 1984 or 1985. Moreover, "senior Pakistani politicians" told a Canadian parliamentary committee in June 1998 that Pakistan had "reached the nuclear 'threshold' by 1984-85." According to former Pakistani Strategic Plans Division (SPD) official Feroz Khan, Pakistan developed a nuclear device suitable for explosive testing by the end of 1984, but it "was still a large bomb that could be delivered only by a C-130 cargo aircraft with no assurance of delivery accuracy."