SEOUL, South Korea – About 12,000 North Korean troops will be sent to Russia, a key United States ally said Thursday, vowing that they would not “stand idly by” in the face of this significant “provocation.”

The new estimate, which South Korea's Defense Ministry told NBC News, comes after the United States, along with Seoul and Kiev, confirmed the development and said any troops deployed against Ukraine were “fair game.”

Pyongyang is expected to bolster the Russian military with its own sizable contingent of special forces, military engineers and artillery troops, South Korean Defense Minister Kim Yong-hyun reported to lawmakers on Thursday, his office told NBC News. The total is expected to reach 12,000, he said, with 3,000 already deployed.

This is consistent with Washington's assessment.

“They are fair game,” National Security Council spokesman John Kirby told reporters on Wednesday, saying the U.S. believed at least 3,000 North Korean soldiers had already arrived by sea in eastern Russia. The soldiers moved earlier this month and are being trained at several Russian military bases, Kirby said.

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