The U.S. is raising the alarm with European Union allies that Russia may be weighing a potential invasion of Ukraine as tensions flare between Moscow and the bloc over migrants and energy supplies.

With Washington closely monitoring a buildup of Russian forces near the Ukrainian border, U.S. officials have briefed EU counterparts on their concerns over a possible military operation, according to multiple people familiar with the matter.

The assessments are believed to be based on information the U.S. hasn¡¯t yet shared with European governments, which would have to happen before any decision is made on a collective response, the people said. They¡¯re backed up by publicly-available evidence, according to officials familiar with the administration¡¯s thinking.