North Korea's feared aim to create nuclear weapons and ballistic missiles was a serious problem for the Clinton Administration. In 1994, North Korea, a signatory of the Nuclear Nonproliferation Treaty, refused to allow international inspectors to review two nuclear waste sites. The inspectors wanted to see if North Korea was in violation of the treaty since they were suspected of reprocessing spent fuel into plutonium, which could be used to manufacture nuclear weapons. Despite diplomatic pressure and repeated warnings by Clinton, North Korea refused to allow the inspections and even raised the prospect of war with South Korea, an ally of the United States. In 1994, Clinton also considered a US military strike on bombing the Yongbyon nuclear reactor. He was advised that if war broke out, it could cost 52,000 US and 490,000 South Korean military casualties in the first three months, as well as a large number of civilian casualties.