SEOUL – 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 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.

During their regular meeting in Seoul on Friday, the chairmen of the joint chiefs of staff from South Korea, the U.S. and Japan discussed North Korea’s deployment of troops to Russia to support its war against Ukraine and Russia’s potential transfer of military technology to North Korea in return.

“They urged the DPRK to immediately cease all unlawful activities to destabilize the Korean Peninsula, the Indo-Pacific, and beyond, and pledged to continue working together to respond to the DPRK’s threats,” the three top military officers said in a joint statement.

DPRK stands for the Democratic People’s Republic of Korea, North Korea’s formal name.

North Korea and Russia have grown sharply closer in recent years, with North Korea supplying thousands of troops and ammunitions to Russia in return for economic and military assistance. Seoul, Washington and their partners worry Russia might provide North Korea with sensitive technologies that can enhance its nuclear and missile programs as well.

The three joint chiefs of staff — South Korea’s Kim Myung-soo, the United States’ Dan Caine and Japan’s Yoshida Yoshihide — discussed various ways to deepen their cooperation in order to ensure peace and stability in the region, according to the joint statement.

Also Friday, the three countries staged a trilateral aerial drill in international waters off South Korea’s southern Jeju island. The training, which involves America’s nuclear-capable B-52H bomber, is meant to improve the deterrence and response capabilities against North Korea’s advancing nuclear and missile programs, according to South Korea’s Defense Ministry.

In recent years, South Korea, the U.S. and Japan have been expanding or restarting their regular military training exercises to cope with North Korea’s enlarging nuclear arsenal. North Korea views such U.S.-led drills as invasion rehearsals and often responds with missile tests.

Lavrov, the Russian foreign minister, was to begin a three-day trip to North Korea on Friday, according to North Korean and Russian state media reports earlier this week.

Russia’s Tass news agency, citing Russian foreign ministry spokeswoman Maria Zakharova, reported Wednesday that Lavrov’s North Korea trip was part of the second round of “strategic dialogue” with his North Korean counterpart Choe Son Hui. Lavrov and Choe’s earlier meeting took place in November in Moscow.

Some South Korean analysts said Lavrov may discuss arranging a visit by North Korean leader Kim Jong Un to Russia.