US President Joe Biden said Thursday that any entry of Russian troops into Ukraine will be treated by the West as "an invasion," as he tried to clarify confusion over an earlier suggestion that a "minor" attack could invite a lesser response.

"If any, any assembled Russian units move across the Ukrainian border, that is an invasion," Biden said, adding he had been "absolutely clear" with his Russian counterpart Vladimir Putin.

"That will be met with severe and coordinated economic response that I have discussed in detail with our allies, as well as laid out very clear with President Putin," Biden told reporters in the White House.

"Russia will pay a heavy price," he said.

However, expanding on comments he made Wednesday that a "minor incursion" by Russia would be treated differently, Biden said that Moscow would not necessarily mount a standard invasion and that "we have to be ready."

US President Joe Biden warned Moscow would not necessarily mount a standard invasion and that 'we have to be ready' US President Joe Biden warned Moscow would not necessarily mount a standard invasion and that 'we have to be ready' Photo: AFP / SAUL LOEB

"That's also not the only scenario that we need to be prepared (for). 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 actions by Russian soldiers not wearing Russian uniforms," Biden said.

He said Moscow also frequently uses cyberattacks.

"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 said.

Ukraine's President Volodymyr Zelensky tweeted Thursday that he wanted "to remind the great powers that there are no minor incursions."

Biden sought to reassure Kyiv directly, saying: "The Ukrainian foreign minister said today that he is confident of our support and resolve -- and he has a right to be."