The development came Friday as Russian Foreign Minister Sergei Lavrov was to travel to North Korea amid booming military and other cooperation between the two countries that have raised concerns among their neighbors.

South Korea, Japan and the United States conducted a joint air drill on Friday involving a U.S. B-52 strategic bomber and fighter jets of the three allies over international waters, the South's defence ministry said.

The Joint Chiefs of Staff of South Korea, the United States and Japan pledged to continue working together toward the complete denuclearization of North Korea, amid discussions about a possible shift in the role of U.