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

Visit by General Dan Caine, top US military officer, comes amid growing calls in Washington for South Korea to shoulder more defence costs.

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

Russian Foreign Minister Sergey Lavrov flew to North Korea's eastern Wonsan city on Friday for a meeting with his North Korean counterpart, Choe Son Hui

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.