US president Joe Biden has again called on President Vladimir Putin of Russia to pull back more than 100,000 troops massed near Ukraine’s borders.
And warned that the US and its allies would “respond decisively and impose swift and severe costs” if Russia invades, according to the White House.
According to a readout of the hour-long call, Mr Biden told Mr 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 Mr Biden’s national security adviser, Jake Sullivan, warned that intelligence shows a Russian invasion could begin within days and before the Winter Olympics in Beijing end on February 20.
The Biden administration has been warning for weeks that Russia could invade Ukraine soon, but US officials had previously said the Kremlin would most likely wait until after the Games ended so as not to antagonise China.
Mr Sullivan told reporters on Friday that US intelligence shows Russia could take military action during the Olympics.
Russia denies that it intends to launch an offensive against Ukraine.
Meanwhile, Ukrainian President Volodymyr Zelenskyy tried to project calm as he observed military exercises on Saturday near Crimea, the peninsula that Russia seized from Ukraine in 2014.
“We are not afraid, we’re without panic, all is under control,” he said.
US Secretary of State Antony Blinken said he told his Russian counterpart on Saturday that “further Russian aggression would be met with a resolute, massive and united trans-Atlantic response”.
US Defence Secretary Lloyd Austin and his Russian counterpart, Sergei Shoigu, also held telephone discussions on Saturday.
The Department of Foreign Affairs is advising against all travel to Ukraine and is asking any Irish citizens to leave the country “immediately by commercial means” amid ongoing tensions at the Russian border.
The advice until this afternoon was against non-essential travel to Ukraine, however, a statement issued this afternoon says the department is upgrading the travel advice following “intensive consultations” with EU partners.
The Irish Embassy in Kyiv will remain open with a small number of essential staff remaining as the situation is kept under review.
Before talking to Mr Biden, Mr 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 towards cooling down the tensions.
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 Kyiv, and Britain joined other European nations in urging its citizens to leave Ukraine.
Russia has massed well over 100,000 troops near the Ukraine border and has sent troops to exercises in neighbouring 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 would not say how definitive the intelligence was.
The White House publicly underscored that the US does not know with certainty whether Mr Putin is committed to invasion.
However, US officials said anew that Russia’s build-up 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.
Mr 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.
Mr Biden’s closely watched call with Mr Putin was conducted from Camp David, the White House said.
Mr 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 Anthony Blinken said he told his Russian counterpart on Saturday that “further Russian aggression would be met with a resolute, massive and united transatlantic response”.
Meanwhile, Ukrainian President Volodymyr Zelenskyy tried to project calm as he observed military exercises on 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 defence secretary Lloyd Austin and his Russian counterpart Sergei Shoigu also held telephone discussions on Saturday.
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 Defence 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 3,000 US troops to Poland to reassure allies.
Mr 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.
Mr 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,” he said, adding that “Russia could choose, in very short order, to commence a major military action against Ukraine”.
Mr Sullivan said the scale of such an invasion could range from a limited incursion to a strike on Kyiv.
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.”
Ms 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 it