The US is increasing the estimated number of North Korean troops sent to Russia, fearing an expansion of Moscow's war against Ukraine.

The Pentagon announced that an estimated 10,000 North Korean troops have been sent to Russia to train and fight against Ukraine in “the next few weeks,” significantly increasing the estimated number of soldiers deployed by North Korea and raising fears that the war is spreading could expand in Ukraine as a result of Pyongyang's military intervention.

Some of the 10,000 North Korean soldiers stationed in eastern Russia for training – up from 3,000 US troops last week – had moved closer to the Ukrainian border, Pentagon spokeswoman Sabrina Singh said on Monday.

“Some of these soldiers have already moved closer to Ukraine, and we are increasingly concerned that Russia intends to use these soldiers in combat or to support combat operations against Ukrainian forces in Russia's Kursk Oblast, near the border with Ukraine,” Singh told reporters.

South Korean President Yoon Suk-yeol said the North Korean military operation threatened both his country's national security and the international community, condemning on Tuesday what he called “illegal” military cooperation between Russia and North Korea.

NATO Secretary General Mark Rutte said earlier on Monday that the North Korean deployment represented a “significant escalation” in the Ukraine conflict and was “a dangerous extension of the Russian war.”

“The deepening military cooperation between Russia and North Korea poses a threat to both Indo-Pacific and Euro-Atlantic security,” Rutte told reporters after talks with a South Korean delegation about North Korean operations.

A North Korean Foreign Ministry official did not confirm media reports of a troop deployment to Russia, but said if Pyongyang had taken such actions, he believed it would be in line with international norms.

“Fake News”

Moscow initially dismissed reports of North Korean involvement in its war against Ukraine as “fake news.” However, Russian President Vladimir Putin has not denied that North Korean troops are in Russia, saying it is an internal question how he will implement any partnership deal with Pyongyang.

Russian Foreign Minister Sergei Lavrov ignored Rutte's comments on Monday, pointing out that Pyongyang and Moscow signed a joint security pact last June.

Lavrov also claimed that Western military trainers have long been secretly sent to Ukraine to help its military use long-range weapons from Western partners.

US Defense Secretary Lloyd Austin and Secretary of State Antony Blinken will meet with their South Korean counterparts in Washington, DC later this week to discuss North Korean involvement in Ukraine.

“When we see DPRK troops advancing towards the front, they are co-belligerents in the war,” Singh said, using the acronym for the country's official name – Democratic People's Republic of Korea. It noted that “there will be no restrictions on the use of U.S.-provided weapons by these forces.”

“This is a calculation that North Korea has to make,” Singh said.

South Korea's national intelligence service Yonhap said President Yoon told a Cabinet meeting that the country must “thoroughly examine all options and prepare countermeasures” given North Korea's military cooperation with Russia.

Ukrainian Foreign Minister Andrii Sybiha said Kiev had been warning about the North Korean deployment for weeks and accused the allies of failing to deliver a decisive response.

“The bottom line: listen to Ukraine. The solution: lift restrictions on our long-range strikes against Russia now,” he said on social media.

However, the US has given no indication that it will agree to Ukraine's request to use its weapons to attack deep inside Russia.