SEOUL (Reuters) -Russian Foreign Minister Sergei Lavrov met North Korean leader Kim Jong Un in the coastal city of Wonsan on Saturday, where he described the two nations' relations as "an invincible fighting brotherhood",

Relations between Russia and North Korea have been flourishing 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 Wonsan Kalma resort was opened in a grand ceremony last month by North Korea's leader, Kim Jong Un, who hailed it as a "world-class tourist and cultural destination". The details of how this resort was built have been shrouded in secrecy in a country largely closed to the outside world.

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.

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.

Washington has said the treatment of Glavprodukt, the only U.S. company Moscow has seized, will influence a planned reset of U.S.-Russia relations.