The US and South Korean defence chiefs have called for North Korea to withdraw its troops from Russia, as the US envoy to the UN, Robert Wood, bluntly warned that Pyongyang’s forces entering Ukraine “will surely return in body bags”.

Washington says 10,000 North Korean troops 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.

“I call upon them to withdraw their troops out of Russia,” US defence secretary Lloyd Austin said at the Pentagon on Wednesday, echoing a call made at a joint appearance with his South Korean counterpart, Kim Yong-hyun.

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.

North Korean troops 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, 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 [Jong-un] to think twice about engaging in such reckless and dangerous behaviour,” Wood added.

Despite his call for them to withdraw, Austin said there was a “good likelihood” that Moscow would still proceed.

South Korea’s Kim, speaking through a translator, said he believed 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.

“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 US and South Korean warnings came as North Korea tested what US officials said was an intercontinental ballistic missile (ICBM). Japan’s government said it reached a maximum altitude of more than 7,000 km – a record height – and flew for an unprecedented one hour and 26 minutes.

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

A Ukrainian official told the Associated Press that North Korean troops are now stationed 30 miles (50km) away from the Ukrainian border with Russia. The official was not authorised to disclose the information publicly and spoke on condition of anonymity.

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, said on Wednesday that Pyongyang’s forces were not present on the frontlines, deeming any suggestion to the contrary as “barefaced lies” and accusing Washington and London of “disinformation”.

Miroslav Jenca, the UN’s assistant secretary-general for Europe, meanwhile said the UN had been following reports of the North Korean deployment to Russia with “serious concern”, but could not independently confirm them.

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 Korean foreign minister Choe Son Hui was in Moscow on Wednesday to hold “strategic” talks with Russian counterpart Sergei Lavrov, while Wang Yi – the top diplomat for China, Pyongyang’s chief diplomatic ally – discussed the Ukraine crisis with Russia’s deputy foreign minister in Beijing.

With Agence France-Presse and Associated Press