In a phone call with President Vladimir Putin on Saturday, US President Joe Biden made clear that if Russia invades Ukraine, the US and its allies would respond "decisively and impose swift and severe costs," the White House said. 

According to a readout of the hour-long call, Biden told Putin that an invasion would "produce widespread human suffering and diminish Russia's standing." The US remains committed to diplomacy, but was "equally prepared for other scenarios," according to the White House. 

The two presidents spoke the day after Biden's national security adviser, Jake Sullivan, warned that US intelligence shows that a Russian invasion could begin within days and before the Winter Olympics in Beijing end February 20. 

Ukrainians attend a rally in central Kyiv, Ukraine yesterday. The US has announced plans to evacuate its embassy in the Ukrainian capital. Photo / AP 

The Biden administration has been warning for weeks that Russia could invade Ukraine soon, but US officials had previously said the Kremlin would likely wait until after the Games ended so as not to antagonise China. 

Sullivan told reporters on Friday that US intelligence gleaned show that Russia could take military action the during the Olympics. 

Russia has more than 100,000 Russian troops massed near Ukraine's borders but denies that it intends to launch an offensive against Ukraine. 

Before talking to Biden, Putin had a telephone call with French President Emmanuel Macron, who met with him in Moscow earlier in the week to try to resolve the biggest security crisis between Russia and the West since the Cold War. A Kremlin summary of the call suggested that little progress was made toward cooling down the tensions. 

The closely watched call between Biden and Putin began shortly after 11am and lasted just over an hour, according to the White House. Biden conducted the call from Camp David. 

In a sign that American officials were getting ready for a worst-case scenario, the United States announced plans to evacuate its embassy in the Ukrainian capital, and Britain joined other European nations in urging its citizens to leave Ukraine. 

Russian Defense Minister Sergei Shoigu, right, and British Defense Secretary Ben Wallace, left, prior to their talks in Moscow, Russia on Friday. Photo / AP 

Russia has massed well over 100,000 troops near the Ukraine border and has sent troops to exercises in neighboring Belarus, but denies that it intends to launch an offensive against Ukraine. 

The timing of any possible Russian military action remained a key question. 

The US picked up intelligence that Russia is looking at Wednesday as a target date, according to a US official familiar with the findings. The official, who was not authorised to speak publicly and did so only on condition of anonymity, would not say how definitive the intelligence was. The White House publicly underscored that the US does not know with certainty whether Putin is committed to invasion. 

However, US officials said anew that Russia's buildup of firepower near Ukraine has reached the point where it could invade on short notice. 

A Kremlin statement about the Putin-Macron call referred to "provocative speculations about an allegedly planned Russian 'invasion' of Ukraine." Russia has consistently denied that it plans military action against its neighbour. 

Putin also complained in the call that the United States and Nato have not responded satisfactorily to Russian demands that Ukraine be prohibited from joining the military alliance and that Nato pull back forces from Eastern Europe. 

Biden has said the US military will not enter a war in Ukraine, but he has promised severe economic sanctions against Moscow, in concert with international allies. 

US Secretary of State Antony Blinken said he told his Russian counterpart Saturday that "further Russian aggression would be met with a resolute, massive and united trans-Atlantic response." 

Meanwhile, Ukrainian President Volodymyr Zelenskyy tried to project calm as he observed military exercises Saturday near Crimea, the peninsula that Russia annexed from Ukraine in 2014. 

"We are not afraid, we're without panic, all is under control," he said. 

US Defense Secretary Lloyd Austin and his Russian counterpart, Sergei Shoigu, also held telephone discussions on Saturday. 

US Secretary of State Antony Blinken speaks about Ukraine during his visit to Nadi, Fiji on Saturday. Photo / AP 

UK troops that have been training the Ukrainian army also planned to leave the country. Germany, the Netherlands and Italy called on their citizens to leave as soon as possible. A State Department travel advisory on Saturday said most American staff at the Kyiv embassy have been ordered to leave and other US citizens should depart the country as well. 

Further US-Russia tensions arose on Saturday when the Defense Ministry summoned the US embassy's military attache after it said the navy detected an American submarine in Russian waters near the Kuril Islands in the Pacific. The submarine declined orders to leave, but departed after the navy used unspecified "appropriate means," the ministry said. 

Adding to the sense of crisis, the Pentagon ordered an additional 3000 US troops to Poland to reassure allies. 

Biden's national security adviser, Jake Sullivan, said Americans in Ukraine should not expect the US military to rescue them in the event that air and rail transportation is severed after a Russian invasion. 

Several Nato allies, including Britain, Canada, Norway and Denmark, also asked their citizens to leave Ukraine, as did non-Nato ally New Zealand. 

Anna Pylypivna, one of the few residents who did not abandon their homes in the frontline village of Novooleksandrivka, eastern Ukraine pictured on February 7. Photo / AP 

Sullivan said Russian military action could start with missile and air attacks, followed by a ground offensive. 

"Russia has all the forces it needs to conduct a major military action," Sullivan said, adding that "Russia could choose, in very short order, to commence a major military action against Ukraine." He said the scale of such an invasion could range from a limited incursion to a strike on Kyiv, the capital. 

Russia scoffed at the US talk of urgency. 

"The hysteria of the White House is more indicative than ever," said Maria Zakharova, a Russian Foreign Ministry spokeswoman. "The Anglo-Saxons need a war. At any cost. Provocations, misinformation and threats are a favourite method of solving their own problems." 

Zakharova said her country had "optimised" staffing at its own embassy in Kyiv in response to concerns about possible military actions from the Ukrainian side. 

In addition to the more than 100,000 ground troops that US officials say Russia has assembled along Ukraine's eastern and southern borders, the Russians have deployed missile, air, naval and special operations forces, as well as supplies to sustain a war. This week, Russia moved six amphibious assault ships into the Black Sea, augmenting its capability to land marines on the coast. 

Sullivan's stark warning accelerated the projected time frame for a potential invasion, which many analysts had believed was unlikely until after the Winter Olympics in China end on February 20. Sullivan said the combination of a further Russian troop buildup on Ukraine's borders and unspecified intelligence indicators have prompted the administration to warn that war could begin any time. 

"We can't pinpoint the day at this point, and we can't pinpoint the hour, but that is a very, very distinct possibility," Sullivan said. 

Ukrainians attend a rally in central Kyiv, Ukraine on Saturday during a protest against the potential escalation of the tension between Russia and Ukraine. Photo / AP 

Biden has bolstered the US military presence in Europe as reassurance to allies on Nato's eastern flank. The 3000 additional soldiers ordered to Poland come on top of 1700 who are on their way there. The US Army also is shifting 1000 soldiers from Germany to Romania, which like Poland shares a border with Ukraine. 

Russia is demanding that the West keep former Soviet countries out of Nato. It also wants Nato to refrain from deploying weapons near its border and to roll back alliance forces from Eastern Europe — demands flatly rejected by the West. 

Russia and Ukraine have been locked in a bitter conflict since 2014, when Ukraine's Kremlin-friendly leader was driven from office by a popular uprising. Moscow responded by annexing the Crimean Peninsula and then backing a separatist insurgency in eastern Ukraine, where fighting has killed over 14,000 people. 

A 2015 peace deal brokered by France and Germany helped halt large-scale battles, but regular skirmishes have continued, and efforts to reach a political settlement have stalled. 

- by Jim Heintz, Aamer Madhani at Associated Press