WASHINGTON (Kyodo) -- The top national security officials of Japan, the United States and South Korea on Friday expressed "grave concern" about North Korea's recent deployment of troops to Russia, possibly to join its war against Ukraine, the White House said.

It said Jake Sullivan, U.S. President Joe Biden's national security adviser, agreed in Washington with his Japanese and South Korean counterparts, Takeo Akiba and Shin Won Sik, respectively, that such actions could "expand the security implications" of the war beyond Europe and into the Indo-Pacific region.

On the eve of their discussions, an official said one of the major objectives of the meeting was to maintain trilateral cooperation, which has deepened under the Biden administration, regardless of who wins the U.S. presidential election on Nov. 5.

While confirming their resolve to "institutionalize efforts to build a new era of trilateral partnership," the White House said the security advisers discussed next steps in the three-way cooperation, including reinforcing supply chains for critical minerals.

Officials have said the three countries are planning to organize a trilateral summit by the end of this year, before Biden leaves office.

The advisers also reaffirmed their countries' strong opposition to any attempts to alter the status quo by force in Indo-Pacific waters, White House national security spokesman John Kirby said, suggesting that China was among the topics under discussion.

Kirby told reporters that the three-way partnership will be "a force for peace and stability in the Indo-Pacific and beyond for years to come."

On North Korea, Kirby said its troop deployment is the latest in a series of concerning signs of strengthening military ties between Pyongyang and Moscow.

He declined to disclose intelligence assessments shared by the security advisers, adding that the United States and its allies are still trying to find out more about what North Korea and Russia are planning to do.

"It is possible that there are now more than 3,000 troops from North Korea that have been dispatched to Russia for outfitting and for training," he said. "We're looking into reports that the number could be north of that. I can't give you a specific estimate at this time."

On Friday, Ukrainian President Volodymyr Zelenskyy said in a post on the X social media platform that based on intelligence, the first batch of North Korean troops is likely to be deployed by Russia to combat zones as early as Sunday.

Zelenskyy called on the international community to take note of "North Korea's actual involvement in combat," a development that clearly shows Russia's intention to continue the war in Ukraine.

A Ukrainian source has said the North Korean military units dispatched to Russia are mainly composed of infantry, artillery and engineers.

Ukraine's military intelligence service said Thursday that some of the soldiers trained in Russia had moved to the country's western region of Kursk near the Ukrainian border.

South Korea's spy agency said last week that North Korea had shipped about 1,500 special forces troops to Vladivostok in the Russian Far East earlier in October.

According to the Ukrainian military source, however, the number is estimated to be fewer than 100, and most of the soldiers in the North Korean units are not believed to possess special skills.

The White House and the Japanese government said Sullivan, Akiba and Shin also held separate one-on-one meetings.

During his talks with Akiba, Sullivan said the United States looks forward to working with the team of Japanese Prime Minister Shigeru Ishiba, who took office in early October, to continue boosting the bilateral alliance that has become global in nature, the White House said.