U.S. President Joe Biden on Wednesday said he had warned Russian president Vladimir Putin of unparalleled sanctions if Moscow attacked Ukraine.

"I made it very clear, if in fact he invades Ukraine, there will be severe consequences, severe consequences, economic consequences like none he's ever seen or ever have been seen," Biden told journalists at the White House.

Biden said he was "absolutely confident" Putin understood the position of the U.S. after their discussions.

Biden's statement comes a day after he and Putin held virtual talks over a range of issues, including the situation about Ukraine.

Biden added that any Russian attack on Ukraine would result in a stronger U.S. military presence on the territory of NATO allies in eastern Europe.

"We would probably be required to reinforce our presence in NATO countries to reassure particularly those in the eastern flank. In addition to that, I made it clear that we would provide a defensive capability to the Ukrainians as well."

However, deploying U.S. troops to defend Ukraine from Russia, Biden added, is not a possibility as the country is not part of NATO.

"The idea the United States is going to unilaterally use force to confront Russia invading Ukraine is not on the cards right now."

"We have a moral obligation and a legal obligation to our NATO allies under Article Five. It's a sacred obligation. That obligation does not extend to Ukraine."

(With input from agencies)