Russia is amassing more troops on Ukraine's border, the US Secretary of State Antony Blinken warned on Friday.
Speaking at a news conference in Melbourne, Mr Blinken said an invasion of Ukraine could begin at any time, including during the Beijing Winter Olympics.
He also confirmed that Washington was continuing to "draw down" its embassy in Ukraine amid the simmering diplomatic tensions.
"Simply put, we continue to see very troubling signs of Russian escalation," he said, adding that the United States "would strongly prefer to resolve the differences" with Russia "through diplomacy".
His comments come as President Joe Biden urged Americans in Ukraine to immediately leave the country in an interview with NBC News.
The Defence Secretary Ben Wallace is travelling to Moscow on Friday morning to meet his Russian counterpart in an attempt to quell the crisis on the Ukrainian border.
The UK has put 1,000 troops on standby in case of a humanitarian crisis in the east if the current Russian military build-up leads to war.
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Russia has reportedly cancelled plans to block part of the Sea of Azov for an upcoming naval exercise which Ukraine had warned would block its ports, writes James Rothwell.
In a post on Facebook, Andriy Klymenko, of the Institute of Black Sea Strategic Studies, said the manoeuvre had been abandoned, possibly due to foreign pressure.
Russia has not yet commented on the affair and it is unclear why the decision was taken. Similar drills in the Black Sea are still due to go ahead as planned, to the alarm of officials in Kyiv.
Finland will sign a $9.4 billion (￡6.9bn) agreement on Friday to buy US F-35 fighter jets, bringing 64 radar-evading warplanes to a country bordering Russia at a time when Moscow is threatening military action unless the West pulls back its forces in Europe.
Finland, which was historically neutral during the Cold War, is a member of the EU but not of the NATO Western military alliance, though it says it aims to cooperate with NATO and retains the right to join.
It's government said the decision to buy the advanced US jets, announced in December, was part of long-term plans to boost the country's defences and not a response to the ongoing confrontation between the West and Russia over Ukraine.
Agreements finalising the purchase of the jets would be signed on Friday with the US government and the manufacturer, Lockheed Martin Corp, the ministry said.
"Yes, today it will be signed... It is a binding agreement that Finland will purchase the jets," Lauri Puranen, director in charge of the purchase at Finland's ministry of defence said. Finland has previously said the planes would begin to be deployed in 2027.
The first event will see Britain’s Defence Secretary lay a wreath at the Tomb of the Unknown Soldier in Moscow, alongside Lieutenant General Yevgeniy Seleznyov, the Commandant of Moscow.
Four American B-52 long-range bombers have arrived in Britain today, as diplomatic tensions simmer between Russia and the West, writes Lucy Fisher, our Deputy Political Editor.
The iconic US military aircraft are headed for RAF Fairford in Gloucestershire, following the arrival of ground crew and logistics personnel two days earlier, it is understood.
The B-52, also known as the Stratofortress, is a long-range heavy bomber that has been in US military service since 1955. It can carry nuclear or precision guided conventional weapons.
The jets, which took off from their home base US Air Force Minot in North Dakota at 2am before refuelling in Nova Scotia, are part of a long-planned Bomber Task Force mission.
Read the full story here: American B-52 'Stratofortress' bomber planes land in UK
Commercial satellite images published by a private US company show new Russian military deployments at several locations near Ukraine, suggesting that Moscow's force buildup continues amid a flurry of diplomacy aimed at easing the crisis.
Russia is holding joint military exercises in ex-Soviet Belarus as well as naval drills in the Black Sea, part of a surge of military activity near Ukraine that has fuelled fears of an impending invasion. Russia denies any plans to attack.
US-based Maxar Technologies, which has been tracking the buildup of Russian forces for weeks, said images taken on Wednesday and Thursday showed significant new deployments in several locations in annexed Crimea, western Russia and Belarus.
In Crimea, which Russia annexed from Ukraine in 2014, Maxar said it had identified a large new deployment of troops and equipment at Oktyabrskoye airfield north of the city of Simferopol, including 550 troop tents and hundreds of vehicles.
New troops and equipment also arrived near Crimea's Novoozernoye, it said, pointing also to a new deployment near the town of Slavne, on the northwest coast of the peninsula.
In Belarus, the site of major joint drills, Maxar said it had identified a new deployment of troops, military vehicles and helicopters at Zyabrovka airfield near Gomel, less than 25 km from the border with Ukraine.
The Defence Secretary is travelling to Moscow on Friday morning to meet his Russian counterpart in an attempt to quell the crisis on the Ukrainian border.
Ben Wallace will meet the Russian Defence minister, General Sergei Shoigu and General Valery Gerasimov, Chief of the General Staff, reports Dominic Nicholls, our Defence Editor.
The first event will see Britain’s Defence Secretary lay a wreath at the Tomb of the Unknown Soldier in Moscow, alongside Lieutenant General Yevgeniy Seleznyov, the Commandant of Moscow.
One company of Kremlin honour guards and a military band will be in attendance.
The Defence Secretary will travel to Moscow on Friday to meet his Russian counterpart in an attempt to quell the crisis on the Ukrainian border.
The trip will follow the Prime Minister's visits to Brussels and Warsaw on Thursday, where he insisted Vladimir Putin must not be allowed to "bully" eastern Europe as he warned the continent faced its biggest security crisis in decades.
Meanwhile, Foreign Secretary Liz Truss was involved in testy exchanges with Russian counterpart Sergei Lavrov, with no sign of a diplomatic breakthrough being achieved following their meeting in Moscow.
It comes as the Government confirmed preparations to allow the UK to "toughen and expand" its sanctions against Russia have come into force.
The Times reported that Ben Wallace will make clear to Sergei Shoigu, the Russian defence minister, that invading Ukraine would be a "lose-lose" situation.
The UK has put 1,000 troops on standby in case of a humanitarian crisis in the east if the current Russian military build-up leads to war.
French President Emmanuel Macron refused a Kremlin request that he take a Russian Covid-19 test when he arrived to see President Vladimir Putin this week, and was therefore kept at a distance from the Russian leader, two sources in Macron's entourage told Reuters.
Observers were struck by images of Macron and Putin sitting at opposite ends of 4-metre-long (13 ft) table to discuss the Ukraine crisis on Monday, with some diplomats and others suggesting Putin might be wanting to send a diplomatic message.
But the two sources, who have knowledge of the French president's health protocol, told Reuters Macron had been given a choice: either he accepted a PCR test done by the Russian authorities and was allowed to get close to Putin, or he refused and had to abide by more stringent social distancing.
"We knew very well that meant no handshake and that long table. But we could not accept that they get their hands on the president's DNA," one of the sources told Reuters, referring to security concerns if the French leader was tested by Russian doctors.
On Thursday, three days after Macron and Putin had their socially-distanced meeting, the Russian leader received Kazakh President Kassym-Jomart Tokayev. The two men shook hands, and sat close to each other, divided only by a small coffee table.
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