"I have been absolutely clear with President [Vladimir] Putin: if any assembled Russian units will move across the Ukrainian border, that is an invasion," Biden told reporters at the White House.
The US president pointed out that such a development would meet "a severe and coordinated economic response."
Biden also said he already ad discussed the potential measures the United States and its allies would undertake in a case of an invasion and conveyed them to Putin.
"There is no doubt that if Putin makes this choice, Russia will pay a heavy price," he said.
Russia has repeatedly dismissed the United States’ allegations that it is preparing to invade Ukraine, pointing instead to the actual threat posed by NATO's military activity in the region. The Kremlin has said Russia has no intention to attack any country and views the accusations against it as a pretext for deploying additional NATO equipment and infrastructure close to Russia’s borders.