US President Joe Biden said on Thursday that he told Russian President Vladimir Putin that any Russian action into Ukraine would be deemed an invasion, despite previously suggesting that allies were divided on how to respond to any “small incursion.”

“I have made it quite clear to President Putin that there is no misunderstanding. If any assembled Russian forces breach the Ukrainian border, it is considered an invasion “Biden told reporters in the Oval Office.

Such an invasion, according to Biden, would be greeted with a “severe and coordinated response, economic retaliation as negotiated in detail with our partners and as put 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.”

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.