Americans have 48 hours to leave Ukraine or the military WON'T rescue them: US ...

The White House told Americans to leave Ukraine immediately on Friday, as officials warned Russia could launch an attack at any time, beginning with air strikes followed by what they said would be an 'onslaught' on the capital Kiev.

President Joe Biden is due to speak to President Vladimir Putin of Russia on Saturday from his Camp David presidential retreat.

He flew out of Washington on Friday afternoon, after his officials spent the day stepping up their warnings and more troops were dispatched the region.

He did not stop to respond to shouted questions from reporters about the unfolding crisis as he left the Oval Office.

Instead he posed for photographs and selfies with staff in the unseasonal sunshine before boarding Marine One.

He left it to his national security adviser to deliver the administration's dire warning to Americans: Get out immediately because the U.S. will not be coming back to rescue anyone.

'If a Russian attack on Ukraine precedes it is likely to begin with aerial bombing and missile attacks that could obviously kill civilians now with regard to their nationality,' he said.

'A subsequent ground invasion would involve the onslaught of a massive force with virtually no notice communications to arrange a departure could be severed and commercial transit halted.

'No one would be able to count on air or rail or road departures once military action got under way.'

He said the attack could come 'any day now'

The region has been on edge for months as Russia sent more than 120,000 troops to the border with Ukraine.

Moscow denies that it is planning to invade its neighbor and instead said it wants Western guarantees that NATO will not encroach closer to its territory.

The latest escalation came after Biden met with security officials in the White House situation room on Thursday evening and as more Russian troops arrived in the area.

President Joe Biden chatted to staff and posed for photos before leaving Washington for a weekend at Camp David, after officials told Americans to leave Ukraine immediately

A Russian tank takes part in drills in a photograph released by the Russian Defense Ministry

A satellite image taken by Maxar Technologies shows a Russian deployment at Zyabrovka airfield in Gomel, Belarus, less than 15 miles from the border with Ukraine

A Russian video of air defence crews taking up position during joint military exercises with Belarus that began on Thursday, further raising the stakes

Another 130 tons of U.S. weapons - including Javelin anti-tank missiles - arrived in Ukraine on Thursday night in response to the 120,000 Russian troops amassed along the country's border

Putin is planning a 'NUCLEAR strategic exercise', Britain warns as Russia begins military drills with Belarus

Russia is planning to hold a nuclear strategic exercise, UK Defence Secretary Ben Wallace said on Thursday, despite extreme tensions surrounding Ukraine.

Wallace warned that the Kremlin's actions were heading in the wrong direction despite efforts to find a diplomatic solution, declaring intelligence suggested that Russia that Russia was engaging in plans for so-called 'false flag' operations as a pre-text for invading Ukraine, as well as carrying out cyber attacks and other destabilising activity.

'Despite the talking, the direction of travel is in the wrong way,' Wallace told BBC Radio.

'The Russians are still growing their battalion tactical groups... They are planning to start a nuclear strategic exercise soon, and indeed we see more activity elsewhere.'

A Russian defence source also told news agency TASS earlier this year that the nuclear exercise, known as 'Thunder', would involve all three prongs of Russia's strategic nuclear forces, testing firing procedures from land, sea and air.

Nuclear drills are held frequently in Russia, but the decision to proceed with such exercises in the face of the tension circling Ukraine will give world powers more cause for concern.

On Friday the Biden administration stepped up its warnings and surged another 3,000 troops to Poland in support of NATO forces.

White House Press Secretary Jen Psaki said Biden would remain closely engaged with his national security team and foreign partners.

'The president will be at Camp David, which is fully equipped to have engagements of all sorts, including with his national security team or European counterparts,' she said.

The latest U.S. deployments follow the 8,500 troops already on alert and the nearly 3,000 American troops the administration said earlier this month it would send to Poland and Romania.

Sullivan did not mince words for those who choose to remain: 'The president will not be putting the lives of our men and women in uniform at risk by sending them into a war zone to rescue people who could have left now but chose not to.'

About 30,000 Americans are thought to be in Ukraine - although officials say the picture is complicated by the fact that many do not register with the embassy

Sullivan said that the invasion could come before the end of the Beijing Olympics, which conclude next week on Feb. 20. But he said that the U.S. is still not sure whether Putin has put in action a plan to invade.

Asked if Putin risks upsetting Chinese President Xi by moving during the Olympics, Sullivan said: 'Putin will obviously have to decide.'

'As we've said before, we are in the window when an invasion could begin at any time should Vladimir Putin decide to order it.'

'We are not saying that a final decision has been taken by Putin,' he added.

Still, the national security adviser said Washington 'can't pinpoint the day, can't pinpoint the hour' when the Russians could invade Ukraine. He also said he did not know how many Americans remained in Ukraine.

President Joe Biden on Friday met virtually with Canadian Prime Minister Justin Trudeau,