Top South Korean, U.S. and Japanese military officers urged North Korea to cease all unlawful activities that threaten regional security, as the three nations flew advanced warplanes for a joint exercise in a show of force against the North.

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 fishermen’s boats had drifted into South Korean waters, officials said. The South’s new president has been trying to improve ties with North Korea.

South Korea's chairman of of the joint chiefs of staff hosted his counterparts from the U.S. and Japan in Seoul on Friday.

South Korea on Wednesday repatriated six North Koreans across the maritime border in the East Sea, months after they drifted into southern waters and were rescued.