US Secretary of State Antony Blinken warned Thursday that "any" Russian troops entering Ukraine would trigger a response, after President Joe Biden raised eyebrows by saying a "minor" incursion could invite a lesser response.

"We have been very clear throughout if any Russian military forces move across the Ukrainian border and commit new acts of aggression against Ukraine, that will be met with a swift, severe response from the United States and our allies and partners," Blinken told reporters in Berlin.