However, relations worsened when South Korean president Lee Myung-bak adopted a more hard-line approach and suspended aid deliveries pending the de-nuclearization of the North. In 2009, North Korea responded by ending all of its previous agreements with the South. It deployed additional ballistic missiles and placed its military on full combat alert after South Korea, Japan and the United States threatened to intercept a Unha-2 space launch vehicle. The next few years witnessed a string of hostilities, including the alleged North Korean involvement in the sinking of South Korean warship ''Cheonan'', mutual ending of diplomatic ties, a North Korean artillery attack on Yeonpyeong Island, and growing international concern over North Korea's nuclear program.