The US is relocating its embassy from Ukraine’s capital because of a “dramatic acceleration in the build-up of Russian forces” while the European Union makes plans to switch energy supplies and house refugees.

The moves come as US officials warn that Moscow could be poised to launch an attack on Ukraine, including on the capital Kyiv.

Russia’s joint exercises with ally Belarus and other drills near Ukraine have fanned fears Russia might invade, something Moscow has repeatedly denied.

Ukraine’s President Volodymyr Zelenskiy said he had heard that Wednesday could be the day of a Russian invasion and would proclaim it a day of Ukrainian national unity instead.

Mr Zelenskiy, who has tended to play down suggestions that an attack is imminent, did not say who had suggested the date of February 16.

However, several US news organisations reported last week that Washington believed that was the date when Russian forces would be ready if President Vladimir Putin gave the order to invade.

“They tell us February 16 will be the day of the attack. We will make it a day of unity,” Mr Zelenskiy said in a video address to the nation.

“They are trying to frighten us by yet again naming a date for the start of military action.”

American Secretary of State Antony Blinken said just before 7am Tuesday (Australian time) the US was moving operations from Kyiv to the western city of Lviv because of fears for the safety of embassy staff.

Most embassy staff have already been ordered to depart Ukraine and US citizens have been advised to leave the country.

“My team and I constantly review the security situation to determine when prudence dictates a change in posture,” Mr Blinken said.

Meanwhile, Russia has suggested it was ready to keep talking to the West to try to defuse the security crisis.

On Tuesday morning, Mr Putin was shown asking his foreign minister, Sergei Lavrov, whether there was a chance of an agreement to address Russia’s security concerns, or whether it was just being dragged into tortuous negotiations.

“We have already warned more than once that we will not allow endless negotiations on questions that demand a solution today,” Mr Lavrov replied.

But he added: “It seems to me that our possibilities are far from exhausted … At this stage, I would suggest continuing and building them up.”

Washington has said Russia could invade Ukraine “any day now” and British Prime Minister Boris Johnson called the situation “very, very dangerous”.

Early Tuesday, Mr Johnson tweeted: “We are on an edge of a precipice but there is still time for President Putin to step back.”

Foreign Secretary Liz Truss wrote that “latest information suggests Russia could invade at any moment”.

Tweet from @BorisJohnson

Russia has positioned more than 100,000 troops near to Ukraine’s borders but denies planning to invade, accusing the West of hysteria.

The EU official said US talks with Russia were “not yielding a lot” but that dialogue with Mr Putin was still open through the leaders of Germany and France.

The official, who spoke under condition of anonymity, said the harsher the EU sanctions against Russia would be in case of a military invasion, the bigger the hit for the bloc should Moscow retaliate.

The official said 40 per cent of EU gas came from Russia. The bloc is in talks with Norway and Qatar, among others, about increased energy supplies if needed.

The official also said the EU was looking at how quickly Russia could switch its energy supplies to China should sales to Europe be cut.

The EU was also preparing for refugee arrivals should Russia invade Ukraine, they said.

The source said the EU was expecting to decide on further macroeconomic support for Ukraine and that Kyiv was seeking more political support.

Some in the EU wanted to impose strong sanctions to discourage Russia from any attack. But others said that would amount to an escalation of tensions and that the bloc should react later only if needed, the official said.

Earlier in the day, the Group of Seven large Western economies (G7) had warned Russia of “massive” economic consequences if it did invade, and promised Kyiv swift support.

Ukraine’s ambassador to Britain backtracked on remarks suggesting that Kyiv would reconsider its attempt to join NATO – one of Russia’s primary concerns – but did say that other concessions could be on offer.

“We are not a member of NATO right now and to avoid war we are ready for many concessions and that is what we are doing in conversations with the Russians,” Vadym Prystaiko told the BBC in a clarification.

“It has nothing to do with NATO, which (membership application) is enshrined in the constitution.”

The Kremlin said that if Ukraine renounced its aspiration to join the Western military alliance, it would significantly help address Russia’s concerns.

Moscow has made clear it sees the former Soviet republic’s quest for closer ties with the West, notably through NATO, as a threat.

-with AAP