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, twice the amount of a previous estimate.

Pyongyang had promised to provide a total around 10,000 troops, and their deployment was expected to be completed by December, the lawmakers told journalists.

Both Moscow and Pyongyang have denied report about North Korean soldiers heading to the battlefield which have also been made by Ukraine.

Ukrainian President Volodymyr Zelenskiy called on allies on Tuesday to respond to evidence of North Korean involvement in Russia's war.

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

Seoul's National Intelligence Service said on Friday the North had sent around 1,500 soldiers to Russia by ship.