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.

South Korea returned on Wednesday six North Koreans who had become stranded at sea earlier this year after their vessels had drifted south, Seoul's Unification Ministry said on Wednesday.