US defence secretary Lloyd Austin called the deployment a "next step" after North Korea previously provided Russia with arms, adding that Pyongyang could face consequences for aiding Moscow directly.

He did not provide details, saying analysts were assessing the situation.

Speaking after South Korea also said its reclusive neighbour had sent 3,000 troops to Russia, Mr Austin told reporters while on a visit to Rome that the US is "seeing evidence" of the deployment.

White House national security spokesman John Kirby has said that the US believes the soldiers travelled by ship to Vladivostok, Russia's largest Pacific port, in early to mid-October.

"These soldiers then travelled onward to multiple Russian military training sites in eastern Russia, where they are currently undergoing training," Mr 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."

Mr Kirby said they could go to western Russian before engaging in combat against Ukraine's forces.

Meanwhile, Mr Austin said if the North Korean troops are "co-belligerents" with the intention of supporting Russia in Ukraine then that is a "very, very serious issue".

He added: "It will have impacts not only in Europe, it will also impact things in the Indo-Pacific."

South Korea's spy chief earlier told legislators that the North Korean troops are in Russia receiving training on drones and other equipment before being deployed to battlefields in Ukraine.

South Korean intelligence officials believe Pyongyang is planning to send 10,000 soldiers to Russia by December.

Last week, South Korea said Pyongyang had sent 1,500 special forces - with Russian military uniforms, weapons and forged IDs - to Russia for training ahead of a likely mission to fight against Ukraine.

It came as Ukrainian President Volodymyr Zelenskyy said his government had intelligence that 10,000 North Korean soldiers were being prepared to join invading Russian forces.

The UK government has also said it is "highly likely" soldiers from North Korea are "supporting Russia's war of aggression".

Russia and North Korea have denied any troop movements have taken place.

The countries have boosted their co-operation in the past two years and in June they signed a major defence deal requiring both countries to use all available means to provide immediate military assistance if either is attacked.

South Korean officials are concerned that Russia may reward North Korea for providing troops by giving it sophisticated weapons that could boost its nuclear and missile programmes.

Seoul said on Tuesday it would consider supplying weapons to Ukraine in response to the reported troop dispatch.

South Korea has previously 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.

NATO secretary-general Mark Rutte said on Tuesday that North Korea's decision to send troops to Ukraine would mark a "significant escalation". He added that he had asked South Korea's president to send experts to Brussels next week to brief the military alliance.

In a further development, Germany said it had summoned North Korea's top diplomat in Berlin over the troop deployment claims.

North Korea has 1.2 million troops, one of the largest standing armies in the world, but it has not 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 experience.

They also say Pyongyang wants Russia's economic support and its help to modernise North Korea's outdated conventional weapons systems as well as its high-tech weapons technology transfers.