U.S. President Joe Biden has said Ukraine should strike back if North Korean troops cross into Ukraine.

According to the White House press corps, when asked on Tuesday about the presence of North Korean troops in Russia, Biden told reporters he was “concerned.”

Asked if Ukraine should strike back, Biden reportedly said, “If they cross into Ukraine, yes.”

Pentagon press secretary Pat Ryder said during a press briefing Tuesday that North Korea has sent about ten-thousand troops to eastern Russia for training, and that these troops will probably augment Russian forces near Ukraine over the next several weeks.

Ryder said a small number of North Korean troops are already in the Kursk region, with a couple of thousand more due to arrive shortly.