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, the Bloomberg sad referring to its sources.

"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," the message reads.

It is noted the assessments are believed to be based on information the U.S. hasn't yet shared with European governments. They're backed up by publicly-available evidence, according to officials familiar with the administration's thinking.

At the same time, Russia says it doesn't intend to start a war with Ukraine now, though Moscow should show it's ready to use force if necessary. An offensive is unlikely as Russian troops would face public resistance in Kyiv and other cities.

As previously reported, the latest movement of Russian troops and tanks towards Ukraine prompted CIA Director William Burns to visit Moscow this month.