North Korea sent at least 3,000 troops to eastern Russia earlier this month, a White House official confirmed, raising a "highly concerning" probability that they could engage in combat to support Russia's war in Ukraine after finishing training.

National Security Communications Advisor John Kirby offered the US' assessment that Pyongyang moved those troops to Russia by ship during a period from early- to mid-October, but stressed that Washington does not yet know what the troops' mission is, Yonhap news agency reported.

The official also warned that should North Korean troops deploy to fight against Ukrainian forces, they could become "fair targets," and that there could be casualties among them.

"We assess that these soldiers travelled by ship from the Wonsan area in North Korea to Vladivostok, Russia. These soldiers then travelled onward to multiple Russian military training sites in eastern Russia where they are currently undergoing training," Kirby told a press briefing on Wednesday, referring to the North's eastern coastal city and Russia's Far Eastern city.

"We do not yet know whether these soldiers will enter into combat alongside the Russian military, but this is certainly a highly concerning probability. After completing training, these soldiers could travel to western Russia and then engage in combat against the Ukrainian military," he added.

Kirby noted that the North Korean soldiers in point are currently getting basic combat and familiarisation training at three Russian sites.

The US' analysis meshes with the assessment by Seoul's key intelligence agency that about 3,000 North Korean soldiers were thought to have been sent to Russia, with a total of 10,000 troops expected to be deployed by December.

In the coming days, Washington plans to announce a "significant" sanctions tranche targeting the enablers of Russia's war in Ukraine located outside of Russia, Kirby stressed.

"Russia's cooperation with the North Korean military violates multiple UN Security Council resolutions, which prohibit the procurement of arms from North Korea and military arms training," he said.

"This move is likewise a violation."

The official pointed out that if Russia is forced to turn to North Korea for military manpower, it would demonstrate an "unprecedented" level of direct military cooperation between the two countries with security implications for Europe and the Indo-Pacific.

Kirby warned of potential North Korean casualties that could occur should the troops engage in combat.

"If they do deploy to fight against Ukraine, they're fair targets, and the Ukrainian military will defend themselves against North Korean soldiers the same way they are defending themselves against Russian soldiers," he said.

"So the possibility that there could be dead and wounded North Korean soldiers from fighting against Ukraine is absolutely real if they get deployed."

The confirmation came after Defence Secretary Lloyd Austin told reporters in Rome that the US saw evidence that Pyongyang had dispatched soldiers to Russia.

"Our analysts are ... they continue to look at this. Now we are seeing evidence that there are North Korean troops that have gone to Russia," Austin said during press availability.

"What exactly they are doing is left to be seen. These are things that we need to sort out."

Asked to elaborate on the intention behind the troop deployment and what North Korea might get from Russia in return, Austin said that the US is trying to gain "better fidelity" on those issues.

"Why are the troops there? We'll continue to pull this thread and see what happens here," he said.

"If they're co-belligerents, if they intend to participate in this war on Russia's behalf, that is a very, very serious issue."

Despite Seoul's confirmation of North Korea's deployment last week, Washington had not confirmed it, stoking speculation that there could be a gap in intelligence analysis between the two allies.

Meanwhile, Maria Zakharova, the spokesperson for Russia's Foreign Ministry, dismissed South Korea's intelligence analysis on North Korea's troop dispatch as "fake news".

She also warned that Seoul could face "security consequences" should it get involved in the Ukrainian crisis, as Seoul has vowed to take "phased" measures in response to deepening military cooperation between Pyongyang and Moscow, including its possible consideration of arms support to Ukraine.

"They should think about the security consequences if it gets involved in the Ukrainian crisis," she said during a press briefing.

"The Russian Federation will react on those aggressive steps, if our citizens are under threats, under peril."