WASHINGTON (BLOOMBERG) - The US believes Russia could take offensive military action or attempt to spark a conflict inside Ukraine as early as next week, before the Winter Olympics in Beijing wrap up, National Security Adviser Jake Sullivan said.

Conflict "could begin during the Olympics despite a lot of speculation that it could only happen after" the Winter Games end, Mr Sullivan told reporters at the White House on Friday (Feb 11).

"What we can say is that there is a credible prospect that a Russian military action would take place even before the end of the Olympics."

"Our view is that we do not believe he has made any kind of final decision, or we don't know that he has made any final decision," Mr Sullivan added, referring to Russian President Vladimir Putin.

As tensions escalate, President Joe Biden and Mr Putin will speak on Saturday, according to US and Russian officials.

The call will take place on Saturday morning, according to a US official. It will be the first direct conversation between the two leaders since Dec 30.

In Fiji on an official visit, US Secretary of State Antony Blinken said he will speak with Russian Foreign Minister Sergei Lavrov on Saturday, where he plans to underscore the “unity and resolve” of allies and partners as tensions surrounding Ukraine escalate.

“We continue to see very troubling signs of Russian escalation including new forces arriving around Ukraine’s borders,” Mr Blinken said at a press conference. 

“We don’t know if President Putin has made that decision” but he has put in place capacity to do so, Mr Blinken said. 

A Russian assault would likely begin with aerial bombing and missile attacks that would cause widespread civilian casualties, Mr Sullivan said, encouraging Americans in Ukraine to leave as soon as feasible.

The US military would not go into Ukraine to evacuate Americans in case of conflict, he said. "I can't obviously predict what the exact shape or scope of the military action will be," Mr Sullivan added.

"As I said before, it can take a variety of forms. It could be more limited, it could be more expansive, but there are very real possibilities that it will involve the seizure of a significant amount of territory in Ukraine and the seizure of major cities including the capital city."

Stocks fell and oil surged to the highest since 2014 as tensions continued to mount. The S&P 500 slid 2 per cent and the Nasdaq 100 dropped more than 2 per cent.

Russia has repeatedly rejected charges it plans to invade Ukraine, accusing Nato of threatening its security by considering Ukrainian membership in the military alliance.

On Thursday, tens of thousands of Russian troops began exercises with the Belarus military just miles from the Ukraine border.

Russia's foreign ministry spokeswoman Maria Zakharova hit back at the US comments on Telegram. "The hysteria of the White House is more revealing than ever," she said.

"The Anglo-Saxons need a war. At any price. Provocations, misinformation and threats are a favourite method of solving one's own problems."

The actions by Russia could include causing a provocation in the Donbas region, where Ukraine's military has been fighting for years against separatists backed by Moscow, or attacking the country's capital, Kyiv, officials familiar with the matter said, asking not to be identified speaking about such a sensitive topic. They said any action could start as soon as Tuesday and stressed Putin's final intentions were not known.

Either move would represent a dramatic escalation after months of tensions over Russia's military buildup near the Ukraine border. The US and other nations estimate Russia now has 130,000 troops in the area. Any action inside Ukraine risks setting off the biggest conflict on European territory since World War II.

The US on Friday ordered an additional 3,000 troops from the 82nd Airborne at Fort Bragg, North Carolina to deploy to Nato ally Poland, according to a US official, bringing the total number of US forces there to 5,000.

Mr Sullivan said US troops stationed in Nato nations are there for defensive purposes only. More than 80,000 US troops are already in Europe, another US official said.

Mr Sullivan warned that Russia would confront a Nato alliance "ready to respond decisively" should it take military action and that the costs on Moscow would be long-lasting.

"If Russia proceeds its long term power and influence will be diminished, not enhanced by an invasion," he said.

The biggest unknown remains the intentions of Mr Putin himself. The US does not know whether Mr Putin has decided to implement such plans, people familiar with the matter said. In addition, the US has not shared evidence underlining the assessments, they said.

"We are trying to stop a war, to prevent the war, to avert a war," Mr Sullivan said. "And all we can do is come here in good faith and share everything that we know to the best of our ability while protecting sources and methods."

US and Russian military leaders spoke by phone earlier Friday, Russia's Interfax reported. The US shared its assessment with key allies on Friday, according to multiple people familiar with the matter. On Friday, several countries moved to withdraw embassy staff from Kyiv, as well as advise citizens to avoid Ukraine and, if there, to leave the country.

US told allies in November that Russia had plans to build up troop numbers and military capabilities near Ukraine's borders. Much of that assessment has come to pass.

Officials have also reiterated they don't believe Putin has made a final decision on Ukraine. Recent US warnings - including that Russia planned to fabricate an attack on pro-Russia separatists to justify an attack - have also not been accompanied by public details of the intelligence behind them.

Mr Biden is likely to spend much of his weekend focused on the Ukraine crisis. The president is travelling to Camp David, Maryland, where he's expected to be in talks with his national security team and European counterparts, White House Press Secretary Jen Psaki said.