London hosted a meeting for foreign ministers from the Group of 8. The group issued a statement condemning "in the strongest possible terms" North Korea's development of nuclear weapons and ballistic missiles.
The G8 consists of Britain, Canada, France, Germany, Italy, Japan, the United States and Russia.
North Korea is said to have missiles ready for launch. A government statement published by South Korean news agency Yonhap warned of imminent war.
"War can break out any moment and what remains to be done is merciless punishment of the enemies," the English-language statement read.
North Korea's war rhetoric has escalated since the February underground nuclear test, said to be part of an anti-U.S. agenda. The U.S. military has responded defensively, though officials said the latest crisis is part of a North Korean pattern.
"We have seen this kind of bellicose rhetoric, these kinds of provocative statements consistently," White House spokesman Jay Carney said earlier this week.
G8 foreign ministers said, in a joint statement, they were concerned not only by North Korea's human rights record, but its recent defiance of the international community.
"Ministers confirmed their commitment to the goal of lasting peace and the verifiable denuclearization of the Korean Peninsula in a peaceful manner," the joint statement said. "They condemned North Korea's current aggressive rhetoric and confirmed that this will only serve to further isolate North Korea."