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.

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 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.