WASHINGTON (AP) — The U.S. said Wednesday that 3,000 North Korean troops have deployed to Russia and are training at several locations, calling the move very serious and warning that those forces will be “fair game” if they go into combat in Ukraine.

The deployment raises the potential for the North Koreans to join Russian forces in Ukraine and suggests expanded military ties between the two nations as Moscow seeks weapons and troops to gain ground in a grinding war that has stalemated after more than two years.

Defense Secretary Lloyd Austin called it a “next step” after the North has provided Russia with arms, and said Pyongyang could face consequences for aiding Russia directly. His comments were the first public U.S. confirmation of North Korea sending troops to Russia — a development South Korean officials disclosed but was denied by Pyongyang and Moscow.

White House national security spokesman John Kirby said the U.S. believes that at least 3,000 North Korean soldiers traveled by ship to Vladivostok, Russia’s largest Pacific port, in early to mid-October.

“These soldiers then traveled onward to multiple Russian military training sites in eastern Russia, where they are currently undergoing training,” Kirby said. “We do not yet know whether these soldiers will enter into combat alongside the Russian military, but this is certainly a highly concerning probability.”

Kirby said they could go to western Russian and then engage in combat against Ukraine’s forces, but both he and Austin said the U.S. continues to assess the situation.

Exactly what the North Korean troops are doing in Russia was “left to be seen," Austin told reporters in Rome.

He added: “If they’re co-belligerents, their intention is to participate in this war on Russia’s behalf, that is a very, very serious issue, and it will have impacts not only in Europe, it will also impact things in the Indo-Pacific.”

Kirby warned, however, that “I can tell you one thing, though, if they do deploy to fight against Ukraine, they’re fair game.”

He said a key question is what North Korea's leader Kim Jong Un is getting out of this.

Russia and North Korea have sharply boosted their cooperation in the past two years, and in June they signed a major defense deal requiring both countries to use all available means to provide immediate military assistance if either is attacked.

South Korean officials worry that Russia may reward North Korea by giving it sophisticated weapons technologies that could boost its nuclear and missile programs that target South Korea. South Korea said Tuesday it would consider supplying weapons to Ukraine in response to the reported troop dispatch.

South Korea’s spy chief had told lawmakers that 3,000 North Korean troops are now in Russia receiving training on drones and other equipment before being deployed to battlefields in Ukraine.

South Korean intelligence first publicized reports that the Russian navy had taken 1,500 North Korean special warfare troops to Russia this month, while Ukrainian President Volodymyr Zelenskyy has said his government had intelligence that 10,000 North Korea soldiers were being prepared to join the invading Russian forces.

On Wednesday, South Korean National Intelligence Service Director Cho Tae-yong told lawmakers that another 1,500 North Korean troops have entered Russia, according to lawmaker Park Sunwon, who attended Cho's closed-door briefing.

Cho told lawmakers his agency assessed that North Korea aims to deploy a total of 10,000 troops to Russia by December, Park told reporters.

Park cited Cho as saying the 3,000 North Korean soldiers sent to Russia have been split among multiple military bases. Cho told lawmakers that NIS believes they have yet to be deployed in battle, Park said.

Also speaking jointly about the briefing, lawmaker Lee Seong Kweun said the NIS found that the Russian military is teaching those North Korean soldiers how to use military equipment such as drones.

Lee cited the NIS chief as saying Russian instructors have high opinions of the morale and physical strength of the North Korean soldiers but think they will eventually suffer heavy causalities because they lack an understanding of modern warfare. Lee, citing Cho, said Russia is recruiting a large number of interpreters.

Lee said NIS has detected signs that North Korea is relocating family members of soldiers chosen to be sent to Russia to special sites to isolate them. The NIS chief told lawmakers that North Korea hasn't disclosed its troop dispatch to its own people.

NATO Secretary General Mark Rutte on Tuesday said North Korea sending troops to Ukraine would mark a “significant escalation,” and said he asked South Korea’s president to send experts to Brussels next week to brief the military alliance.

Ukraine’s Military Intelligence Directorate head, Kyrylo Budanov, told the online military news outlet The War Zone on Tuesday that North Korean troops were to arrive to Russia's Kursk region on Wednesday to help Russian troops fighting off a Ukrainian incursion.

Last week, South Korea's spy agency said North Korea had sent more than 13,000 containers of artillery, missiles and other conventional arms to Russia since August 2023 to replenish its dwindling weapons stockpiles.

Reports that the North is sending troops to Russia stoked security jitters in South Korea. It has shipped humanitarian and financial support to Ukraine, but it has so far avoided directly supplying arms in line with its policy of not supplying weapons to countries actively engaged in conflicts.

North Korea has 1.2 million troops, one of the largest standing armies in the world, but it hasn’t fought in large-scale conflicts since the 1950-53 Korean War. Experts question how much North Korean troops would help Russia, citing a shortage of battle experiences.

Experts say North Korea wants Russia's economic support and its help to modernize the North's outdated conventional weapons systems as well as its high-tech weapons technology transfers.

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Associated Press writers Kim Tong-hyung in Seoul, Illia Novikov and Hanna Arhirova in Kyiv, Ukraine, Danica Kirka in London and Jari Tanner in Helsinki contributed to this report.