On May 27, 2009 North Korea media declared that the armistice is no longer valid due to the South Korean government's pledge to "definitely join" the Proliferation Security Initiative. To further complicate and intensify strains between the two nations, the sinking of the South Korean warship Cheonan in March 2010, killing 46 seamen, is as of May 20, 2010 claimed by a team of researchers around the world to have been caused by a North Korean torpedo, which the North denies. South Korea agreed with the findings from the research group and president Lee Myung-bak declared in May 2010 that Seoul would cut all trade with North Korea as part of measures primarily aimed at striking back at North Korea diplomatically and financially. As a result of this, North Korea severed all ties and completely abrogated the previous pact of non aggression.