The United States is warning Russia that any further intervention in Ukraine would be viewed as "an invasion of Ukraine."

U.S. Ambassador Samantha Power delivered the warning Friday at a Security Council meeting on Ukraine. It follows recent reports by the West and Ukraine's pro-Western government that accused Moscow of beefing up its military on the border, dispatching what NATO estimates is 20,000 troops to the frontier.

Power said if Moscow wants to send aid to pro-Russian separatists it should be delivered by neutral international organizations — not by Russians.

"Therefore any further unilateral intervention by Russia into Ukrainian territory, including one under the guise of providing humanitarian aid, would be completely unacceptable and deeply alarming — and it would be viewed as an invasion of Ukraine," she said.