Speaking at Yonsei University, Graham Allison, director of Harvard University's Belfer Center for Science and International Affairs, discussed what might happen if terrorists got hold of a North Korean nuclear bomb and used it to attack the United States.
"If you imagine the North Korean regime selling a bomb to some group like al-Qaida, maybe to Iran, maybe to a splinter terrorist group in Pakistan, and they took this nuclear bomb and exploded it in an American city, what's going to happen in North Korea?" Allison asked, Yonhap News reported.
He said the United States would retaliate against the North and the impact would be felt in South Korea because of its proximity.
"By extension, a nuclear bomb that goes loose in North Korea could be the biggest event that ever happened in South Korea even though it didn't explode here," Allison said.
He noted North Korea had been accused of helping Syria build the nuclear reactor that Israel later bombed in 2007.
Allison, a former U.S. assistant secretary of defense and an expert on nuclear terrorism, was in Seoul to meet with South Korean President Lee Myung-bak as a member of an international panel advising Lee on the upcoming Nuclear Security Summit on March 26 and 27.