The United States says an estimated 10,000 North Korean soldiers have been deployed in Russia as the NATO chief calls the move “a significant escalation” in Russia’s war against Ukraine.

The Pentagon said on Monday that it is “increasingly concerned that Russia intends to use these soldiers in combat” in Kursk, a Russian region on the border with Ukraine that was invaded by Kyiv’s forces in August.

NATO Secretary-General Mark Rutte said: “The deepening military cooperation between Russia and North Korea is a threat to both Indo-Pacific and Euro-Atlantic security.”

Ukraine, supported by Western arms shipments, has been battling Russian forces since Moscow launched a full-scale invasion of the Eastern European nation in February 2022. Russia has since captured parts of eastern and southern Ukraine and has made military gains in recent months.

So what impact could the possible presence of North Korean troops in Russia have on the Ukraine war?

What do we know about the North Korean deployment?

The Pentagon and NATO are not the first to confirm the North Korean military presence in Russia. Days before, Ukrainian intelligence said it had recorded the presence of Pyongyang’s soldiers in the Kursk region.

Three weeks ago, South Korean Defence Minister Kim Yong-hyun said it was “highly likely” that six North Korean officers were killed in a Ukrainian strike on October 3 near Donetsk in eastern Ukraine.

On October 18, South Korea’s National Intelligence Service said Russian ships had transferred 1,500 North Korean soldiers to Russia during the second week of October.

In a video address on October 13, Ukrainian President Volodymyr Zelenskyy also alleged North Korea had transferred troops to Russia. In the same video, he urged his allies in the West to lift restrictions that keep Ukraine from using their long-range missiles to strike deep inside Russian territory.

Ukraine, South Korea and the US have not yet offered any evidence to back their statements about the North Korean deployments. However, experts earlier told Al Jazeera that the possibility cannot be ruled out because Russia and North Korea have cooperated militarily over the course of the nearly three years of the war.

The allegations of North Korea deploying its soldiers, which the Kremlin has called fake news, have made it into the news headlines after Moscow and Pyongyang signed a mutual defence pact in June during Putin’s first state visit to the isolated nation in 24 years.

The text of this agreement was not released, but it includes a mutual assistance clause calling on the two countries to provide military assistance should one of them be attacked.

This pact prompted concern from Washington’s allies in East Asia – South Korea and Japan. Pyongyang and Seoul are still technically at war because the 1950-1953 Korean War ended with an armistice and not a peace treaty. More than 50,000 US soldiers are stationed in Japan, and nearly 25,000 are deployed in South Korea.

It is believed Russia could use Ukraine’s Kursk incursion to invoke the mutual assistance clause of the pact.

What have Russia and North Korea said?

Russia had earlier denied the presence of North Korean troops in its territory.

Kremlin spokesman Dmitry Peskov told reporters that “this seems like yet another fake news story.”

However, President Vladimir Putin was asked by a reporter during the BRICS summit on Thursday about satellite imagery showing North Korean troops in Russia. “Images are a serious thing. If there are images, then they reflect something,” Putin responded.

There has not been a public response from North Korea about whether its troops are in Russia.

The US, South Korea and Ukraine have also alleged that North Korea has transferred arms to Russia, but Pyongyang and Moscow have denied this.

According to Russian media reports, North Korean Foreign Minister Choe Son Hui arrived in Russia on Tuesday. It is unclear what will be on Choe’s agenda during the trip.

If confirmed, how would the deployment impact the Ukraine war?

If the presence of North Korean troops in Russia is officially confirmed, it would make Pyongyang the first government to officially send its soldiers to the front.

So far, only Russian and Ukrainian soldiers have fought the war. Mercenaries from all over the world, especially South Asian countries, have joined each side, but they were not sent by their governments.

“Indeed, North Korea is now a full-fledged participant in the Ukraine war rather than merely supporting Russia by supplying artillery shells without any direct involvement in the conflict,” Edward Howell, a lecturer in international relations at the University of Oxford, told Al Jazeera.

However, he said this might not change Moscow’s overall strategy in the war.

“Not least given how the North Koreans, it seems, will be organised in their own battalions and have their own translators,” said Howell, whose research focuses on the politics and international relations of North Korea, the Korean Peninsula and East Asia.

The deployment of North Korean troops could shift the balance on the battlefield during a war of attrition. As of October 1, Russia has lost 654,430 soldiers who have been killed and wounded, according to the Ukrainian military’s General Staff. Russia has not shared any casualty figures, but a Russian media report put the death toll at more than 71,000 soldiers.

While Ukraine enforces conscription, it has been easier for Russians to evade conscription. So North Korean troops could lend strength in numbers to the Kremlin’s war efforts.

The developments in Russia are taking place against a backdrop of escalating tensions between North Korea and South Korea. On October 15, tensions spiked on the Korean Peninsula when North Korea blew up roads that connect it to Seoul.

This came days after North Korea’s Ministry of Foreign Affairs released a statement saying it had detected drones in the skies over Pyongyang and accusing South Korea of sending them.

“What is concerning is whether, in return for the dispatch of North Korean troops, Russia will provide North Korea with any military technology which North Korea could use in any provocations against South Korea,” Howell said.

Will the West allow Ukraine to use long-range weapons against Russia?

Putin has warned the West against allowing Ukraine to use its long-range weapons against Moscow. Putin has hinted at retaliation, raising fears of a major escalation in the war.

During the news briefing on Monday, Pentagon spokesperson Sabrina Singh was asked by a reporter whether there would be limits on which weapons sent by the US that Ukraine could use against North Korean soldiers.

Singh said no, adding that North Korean forces “are co-belligerents in the war, and so they are fighting on these front lines and the Ukrainians are defending their sovereign territory and pushing the Russians back”.

“We’ve made a commitment to Ukraine that we’re going to continue to support them with whatever it takes,” she said.

Howell said: “The possibility for restrictions on long-range missiles to be lifted remains likely as more and more participants engage with the conflict and the need to disrupt Russia’s tactics becomes ever more urgent.”