North Korea has sent troops to Russia, the United States said Wednesday, its first public confirmation of a move that has rattled Western allies and could mark a major escalation of Moscow’s war in Ukraine.

“There is evidence of DPRK troops in Russia,” Defense Secretary Lloyd Austin told reporters in Rome, using the abbreviation for North Korea’s formal name, the Democratic People’s Republic of Korea.

“What exactly they’re doing is left to be seen,” Austin said, adding, “We’re trying to gain better fidelity on it.” It’s a “serious issue,” he said, if North Korea’s “intention is to participate in this war on Russia’s behalf.”

His comments came after South Korea and Ukraine sounded the alarm in recent days, sharing intelligence and voicing dissatisfaction with what they see as a lack of urgency in the response from the U.S. and other Western countries.