"We are in the window. Any day now, Russia could take military action against Ukraine, or it could be a couple of weeks from now, or Russia could choose to take the diplomatic path instead," Sullivan told the "Fox News Sunday" program.
Sullivan made the comments in television interviews after two US officials on Saturday said Russia, which seized Crimea from Ukraine in 2014, has in place about 70 per cent of the combat power it believes it would need for a full-scale invasion of Ukraine.
As Russia masses more than 100,000 troops near the border, Moscow has said it is not planning an invasion but could take unspecified military action if its security demands are not met.
Those include a promise that NATO will never admit Ukraine, a demand the United States and the 30-nation Western security alliance have called unacceptable.
Possible Russian action could include annexing Ukraine's Donbass region, where Russian-backed separatists broke away from Ukrainian government control in 2014, cyber attacks or a full-scale invasion of Ukraine, Sullivan added, saying Russia could act as soon as Monday though it could also be weeks.
"We believe that there is a very distinct possibility that Vladimir Putin will order an attack on Ukraine," Sullivan told ABC's "This Week" program, referring to the Russian leader.
"It could take a number of different forms. It could happen as soon as tomorrow, or it could take some weeks yet. He has put himself in a position with military deployments to be able to act aggressively against Ukraine at any time now," Sullivan added.
Ukraine, while seeking more military aid, has also sought to calm fears of an invasion. Foreign Minister Dmytro Kuleba on Sunday urged people to ignore "apocalyptic predictions," saying his country was strong and had unprecedented international support.
Washington has made clear it would not send US soldiers to defend Ukraine, which is not a member of NATO.
However, Washington has given Kyiv arms and last week said it would send nearly 3000 extra troops nL1N2UD16S to Poland and Romania to shield Eastern Europe from potential spillover from the crisis.
A plane carrying US troops landed in Poland on Sunday, a Reuters witness said. On Wednesday, the Pentagon said around 1700 service members, mainly from the 82nd Airborne Division, would deploy from Fort Bragg, North Carolina, to Poland.
US President Joe Biden sent a message to the troops as he arrived at the White House after a weekend at his Delaware home.
"We're thinking of you," he said, clenching his fist as he spoke to reporters.
Biden said he would not speculate on whether he might need to send more troops to Europe. Asked what Putin is weighing as he decides whether to pursue an invasion, Biden said the Russian president was looking for "things he cannot get."
Biden spoke to French President Emmanuel Macron on Sunday ahead of the French president's visit to Moscow, an effort to bring down the tensions that is coordinated with Washington and Berlin, according to a White House statement.
German Chancellor Olaf Scholz, who will meet Biden at the White House on Monday, signaled on Sunday he was open to deploying more troops to Lithuania to bolster NATO's eastern flank.
15°C