The US and South Korean defense chiefs called Wednesday for North Korea to withdraw its troops from Russia, where Washington says some 10,000 of them have been deployed for possible action against Ukrainian forces.

Russia and North Korea have deepened their political and military alliance as the Ukraine war has dragged on, but sending Pyongyang`s troops into combat against Kyiv`s forces would mark a significant escalation that has sparked widespread international concern, AFP reports.

These North Korean troops -- wearing Russian uniforms and blended in with units of ethnic minorities to try to conceal them -- are expected to go into combat against Ukrainian forces in November, said Ukraine`s UN ambassador Sergiy Kyslytsya.

"I call upon them to withdraw their troops out of Russia," US Defense Secretary Lloyd Austin said at the Pentagon, echoing a call by his South Korean counterpart Kim Yong-hyun, who stood beside him.

Austin said the United States will "continue to work with allies and partners to discourage Russia from employing these troops in combat."

But there is a "good likelihood" that Moscow will still do so, according to the US defense secretary, who said that North Korean forces are being outfitted with Russian uniforms and weapons.

Kim, speaking through a translator, said he believes the North Korean deployment to Russia "can result in the escalation of the security threats on the Korean peninsula."

That is because there is a "high chance" that Pyongyang will ask for technology transfers from Russia to aid its weapons programs -- including on tactical nuclear weapons, intercontinental ballistic missiles and reconnaissance satellites -- in exchange for the deployment of its forces, he said.

But he did not announce a change to Seoul`s longstanding policy that bars it from selling weapons into active conflict zones including Ukraine -- a stance it has stuck to despite calls from Washington and Kyiv to reconsider.

- `Return in body bags` -

"At the current moment, nothing is determined," Kim said when asked if there are plans for South Korea to indirectly supply munitions to Ukraine.

The Pentagon said the previous day that a "small number" of North Korean troops have already been deployed in Russia`s Kursk region, where Ukrainian forces have been conducting a ground offensive since August.

Ukraine`s Kyslytsya said at the United Nations that the North Korean troops -- as many as 12,000 -- are being trained at five grounds in eastern Russia and include 500 officers and three generals. He said all this was "according to available information."

Russia`s President Vladimir Putin has not denied the deployment of North Korean troops to his country but has also refused to confirm it.

Moscow`s UN envoy Vassily Nebenzia on Wednesday said that any claims that Pyongyang`s forces were present on the front lines were "mere assertions."

The White House has said that Pyongyang`s forces would become "legitimate military targets" if they fight against Ukraine, and Austin echoed that stance on Wednesday.

If North Korean troops "are fighting alongside Russian soldiers in this conflict and attacking Ukrainian soldiers, Ukrainian soldiers have the right to defend themselves," Austin said.

They would be "co-belligerents, and you have every reason to believe that... they will be killed and wounded as a result of battle," he added.

Speaking before the UN Security Council, US envoy Robert Wood gave an even more explicit warning, saying if Pyongyang`s forces "enter Ukraine in support of Russia, they will surely return in body bags."

"I would advise Chairman Kim to think twice about engaging in such reckless and dangerous behavior," Wood added.

Pyongyang has denied sending troops to Russia, but its vice foreign minister said that were such a deployment to happen, it would be in line with global norms.

North Korea and Russia are both under UN sanctions -- Pyongyang for its nuclear weapons program, and Moscow for the Ukraine war.