Russia has massed enough troops near Ukraine to launch a major invasion, Washington said on Friday, as it urged all US citizens to leave the country within 48 hours.
An attack by the more than 100,000 Russian troops currently massed next to Ukraine "could occur any day now," White House National Security Advisor Jake Sullivan told reporters in Washington.
It remains unclear, Mr Sullivan said, whether Russian President Vladimir Putin had definitively given an order to start an invasion. Mr Sullivan said he expected US President Joe Biden to seek out a phone call with Mr Putin soon on the crisis.
The build-up of Russian military forces on Ukraine's border matters to Australia despite being half a world away, US Secretary of State Antony Blinken says.
Following, Mr Blinken said the United States would maintain its dual-track approach to Russia and the forces it has amassed at the border "unprovoked".
That strategy involves the US keeping diplomatic dialogue open, while also building deterrences and defences if Russia chooses to invade.
"(We've been) making it clear to Russia that if it chooses the path of renewed aggression, it will face massive consequences," Mr Blinken told reporters in Melbourne on Friday after the fourth Quad alliance meeting.
In an interview with Nine newspapers, Mr Blinken said he would expect Australia to play its role in imposing "massive costs" on Moscow should it invade.
He said the US and its allies were still ironing out what that would entail, but floated the prospect of economic and financial sanctions, export controls, and building up Ukraine's armed forces and NATO defences.
In the meeting with her counterparts from the US, India and Japan, Foreign Minister Marise Payne said she reiterated her "very deep concerns" about the presence of Russia's military on the Ukrainian border.
"We will continue to support our allies and partners to deter this sort of aggression and to raise the costs of this kind of behaviour."
Earlier, as Moscow further stiffened its response to Western diplomacy, commercial satellite images from a private US company showed new Russian military deployments at several sites near the border.
After telling NBC News that things in Ukraine "could go crazy quickly," Mr Biden held a phone call on the crisis with the leaders of Britain, Canada, France, Germany, Poland and Romania, as well as the heads of NATO and the EU.
Following that meeting and with alarm spreading, British Prime Minister Boris Johnson joined a handful of other nations in urging their citizens to leave Ukraine.
Mr Johnson told the meeting that he feared for the security of Europe and stressed the need for "a heavy package of economic sanctions ready to go, should Russia make the devastating and destructive decision to invade Ukraine," his office said.
Moscow, meanwhile, said answers sent this week by the EU and NATO to its security demands showed "disrespect".
Mr Biden met his national security advisers overnight, a source familiar with the meeting said. US officials believed the crisis could be reaching a critical point, with rhetoric from Moscow hardening, six Russian warships reaching the Black Sea, and more Russian military equipment arriving in Belarus, the source said.
"We're in a window when an invasion could begin at any time, and to be clear, that includes during the Olympics," said Secretary of State Antony Blinken. The Beijing Games end on 20 February.
"Simply put, we continue to see very troubling signs of Russian escalation, including new forces arriving at the Ukrainian border."
Russia has denied it plans to invade Ukraine.
