US President Joe Biden said on Thursday that he has made clear to Russian President Vladimir Putin that any Russian movement into Ukraine would be considered an invasion, after he earlier suggested allies were split over how to react to any "minor incursion." "I have been absolutely clear with President Putin, he has no misunderstanding. If any, any assembled Russian units move across the Ukrainian border that is an invasion," Biden told reporters at the White House. Biden said such an invasion would be met by a "severe and coordinated response, economic response as discussed in details with our allies as laid out very clearly with president Putin." He added: "But there is no doubt, let there be no doubt at all, that if Putin makes this choice, Russia will pay a heavy price." Also read: 'No ultimatums': Russia sets out security demands at NATO meeting Russia has massed tens of thousands of troops on its borders with Ukraine, and Western states fear Moscow is planning a new assault on a country it invaded in 2014. Russia denies this. Biden said Washington was on the lookout for other scenarios. "Russia has a long history of using measures other than overt military action to carry out aggression. Paramilitary tactics, so-called gray zone attacks and action by Russian soldiers not wearing Russian uniforms." Biden said other potential covert action by Moscow included cyber attacks as well as what he called the deployment of "little green men in uniform." "We have to be ready to respond to these as well in a decisive and united way with the range of tools at our disposal," Biden added.