North Korea has sent 3,000 troops to Russia to support its war against Ukraine, South Korean lawmakers said on Wednesday after being briefed by the national intelligence agency. That estimate is twice the previous figure.

Pyongyang had promised to provide a total of about 10,000 troops, whose deployment was expected to be completed by December, the lawmakers told journalists.

The latest numbers came after Seoul's National Intelligence Service said on Friday the North had sent some 1,500 special forces to Russia by ship.

"Signs of troops being trained inside North Korea were detected in September and October," Park Sun-won, a member of a parliamentary intelligence committee, said after the briefing.

"It appears that the troops have now been dispersed to multiple training facilities in Russia and are adapting to the local environment."

Ukrainian President Volodymyr Zelenskyy has also accused Pyongyang of preparing to send 10,000 soldiers to Russia, calling on allies on Tuesday to respond to evidence of North Korean involvement in Russia's war.

'Significant escalation' if deployment confirmed

U.S. Defence Secretary Lloyd Austin, speaking to reporters in Rome on Wednesday, said "there is evidence" that North Korean troops are in Russia, and it remains to be seen what they would be doing there.

If the presence of North Korean troops is confirmed, "that would mark a significant escalation," NATO Secretary General Mark Rutte said on Tuesday.

WATCH | U.S. defence secretary speaks of 'evidence' of troop deployment:

'There is evidence' of North Korean troops in Russia, U.S. defence secretary says

U.S. Defence Secretary Lloyd Austin said Wednesday that it's not yet clear what exactly North Korean troops are doing in Russia.

Both Moscow and Pyongyang have denied arms transfers, but have pledged to boost military ties, signing a mutual defence treaty at a summit in June. The Kremlin has dismissed as "fake news" South Korea's claim about the North's troop dispatch.

On Monday, a top U.S. diplomat said Washington was consulting with allies on the implications of North Korean involvement, adding that such a development would be "dangerous and highly concerning" if true.

Pyongyang has not publicly responded to the claims by Seoul and Kyiv, but authorities there made efforts to keep news of the dispatch from spreading, said Lee Seong-kweun, another lawmaker on the South Korean committee.

WATCH | Ukraine says North Korean troops preparing to enter the conflict: 

Unverified video appears to show North Korean troops in Russia

A Ukrainian government agency says North Korea has sent troops to Russia to support its military operation in Ukraine. Unverified video purportedly shows North Koreans picking up Russian uniforms and military gear.

Lee also said the spy agency confirmed that Russia had recruited a "large number" of interpreters for the North Korean soldiers, while training them in use of military equipment, such as drones.

"Russian instructors are assessing that the North Korean military has excellent physical attributes and morale but lacks understanding of modern warfare such as drone attacks," the lawmaker said.

Park said Pyongyang may be seeking in return to prompt intervention by Russia in case of a contingency on the Korean peninsula as well as receive economic assistance.

On Tuesday, the South's presidential office urged an immediate withdrawal of the North's troops from Russia, warning that it may consider supplying lethal weapons to Ukraine if military ties between them went too far.