SEOUL, South Korea — About 12,000 North Korean troops will be sent to Russia, a key U.S. ally said Thursday, pledging that the country “will not stand by and do nothing” in the face of this major “provocation ”.

The new estimate, shared with NBC News by South Korea’s Defense Ministry, comes after the United States joined Seoul and Kyiv in confirming the development and said any troops deployed against Ukraine are “fair game” will be.

Pyongyang is expected to expand Russia’s military with its own sizable contingent of special forces, military engineers and artillery troops, South Korean Defense Minister Kim Yong-hyun told lawmakers on Thursday, his office told NBC News. The total number is expected to reach 12,000, he said, while 3,000 have already been deployed.

That agrees with Washington’s assessment.

“They’re fair game,” National Security Council spokesman John Kirby told reporters on Wednesday. He said the US believes at least 3,000 North Korean soldiers have already arrived in eastern Russia by sea. The soldiers were moved earlier this month and are receiving training at multiple Russian military bases, Kirby said.

“They are fair targets and the Ukrainian army will defend itself against North Korean soldiers the same way they defend themselves against Russian soldiers,” he said. “There could be dead and wounded North Korean soldiers fighting Ukraine.”