President Joe Biden expressed concern Friday about what he called “dangerous and destabilizing cooperation” between North Korea and Russia, as he met with the leaders of South Korea and Japan at the global summit of Asia-Pacific leaders in Peru.

In a joint statement, Biden, Japanese Prime Minister Shigeru Ishiba and President Yoon Suk Yeol of South Korea said they “strongly condemn” the cooperation between North Korea and Russia, including the decision by North Korea to send thousands of troops to Russia to help President Vladimir Putin in his war with Ukraine.

“Deepening military cooperation between the DPRK and Russia, including munitions and ballistic missile transfers, is particularly egregious given Russia’s status as a Permanent Member of the U.N. Security Council,” the three leaders said, using an acronym for North Korea. “We remain resolute as ever in supporting Ukraine as it exercises its inherent right to self-defense as enshrined in Article 51 of the U.N. Charter.”

A senior U.S. official, who spoke on the condition of anonymity to describe the discussions during the 40-minute closed-door session, said that the issue of Russia-North Korea cooperation dominated the discussion among the three leaders.

The president and his aides have said in recent weeks that they are deeply worried about actions by Kim Jong Un, North Korea’s leader, including the dispatching of troops to Russia and new testing of a long-range ballistic missile.

On Tuesday, Biden said those developments must be countered by close coordination with allies in the region. Flanked by Ishiba and Yoon, Biden expressed confidence in security and economic cooperation the three nations announced last year at Camp David.