The Kremlin has
dismissed South Korea's claim about the North's
troop deployment as "fake news" and a North Korean representative to the United Nations in New York called it "groundless rumours" at a meeting on Monday.
Both Moscow and Pyongyang have also denied weapons transfers but they have pledged to boost military ties, signing a mutual defence treaty at a summit in June.
The latest numbers came after Seoul's National Intelligence Service said on Friday the North had sent some
1,500 special forces personnel to Russia by ship.
Ukrainian President Volodymyr Zelenskiy has also accused Pyongyang of preparing to send 10,000 soldiers to Russia. On Tuesday he called on his
allies to respond to evidence of North Korean involvement in Russia's war.
FAMILIES ISOLATED
Lee Seong-kweun, a lawmaker on the South Korean committee, said Pyongyang authorities had tried to keep news of the deployment from spreading.
"There are also signs of North Korean authorities relocating and isolating those families (of the troops) in a certain place in order to effectively control them and thoroughly crack down on the rumours," Lee said, citing the spy agency.
Lee also said the agency confirmed that Russia had recruited a "large number" of interpreters for the North Korean soldiers, while training them in the 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," he said.
"Therefore there could be many casualties if they are deployed to the front lines."
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.
U.S. officials, speaking on condition of anonymity, say Russia has suffered more than 600,000 killed or wounded troops in the war in Ukraine.
Austin said the alleged North Korean deployment could point to a shortage of Russian recruits.
"This is an indication that he (Russian President Vladimir Putin) may be even in more trouble than most people realize," Austin said.
Lawmaker 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 to receive economic assistance.
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Reporting by Hyonhee Shin and Joyce Lee; Editing by Andrew Heavens, Clarence Fernandez and Angus MacSwan