With the United States and Russia at an impasse over the ongoing Ukraine crisis, White House national security adviser Jake Sullivan said Thursday that Washington is "ready either way" for what may come.

"The U.S. does not believe Russia has decided to invade Ukraine but is prepared no matter which path it takes," Sullivan told reporters after a week of talks between the U.S., its Western allies and Russia.

"We're ready to make progress at the negotiating table ... and we're ready to take the necessary and proper steps to defend our allies, support our partners and respond robustly to any naked aggression that might occur."

Russia has amassed tens of thousands of troops on the border with Ukraine as it demands guarantees that NATO will not expand the alliance and include Ukraine.

"The intelligence community has not made an assessment that the Russians have definitively decided to take a military course of action in Ukraine," he said.

"So as things stand right now, Russia has the opportunity to come to the table."