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.

Relations between Russia and North Korea have flourished in recent years, with North Korea supplying troops and ammunition to support Russia's war against Ukraine in return for military and economic assistance.

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.