According to Greenpeace, the United States and USSR were concerned enough by accidents such as the 1961 Goldsboro B-52 crash, the 1966 Palomares B-52 crash and the Thule accident that they agreed to take measures to ensure that a future nuclear accident would not lead the other party to conclude incorrectly that a first-strike was under way. Consequently, on 30 September 1971, the two superpowers signed the "Agreement on Measures to Reduce the Risk of Nuclear War". Each party agreed to notify the other immediately in the event of an accidental, unauthorized or unexplained incident involving a nuclear weapon that could increase the risk of nuclear war. They agreed to use the Moscow–Washington hotline, which was upgraded at the same time, for any communications.