(Al Jazeera) President Joe Biden has urged United States citizens in Ukraine to leave the country immediately, citing the threat of Russian invasion amid the ongoing crisis in Eastern Europe.

“American citizens should leave now,” Biden said in a pre-taped interview with NBC News that was released on Thursday, reiterating a message from last week.

“It’s not like we’re dealing with a terrorist organisation. We’re dealing with one of the largest armies in the world. It’s a very different situation and things could go crazy quickly,” he added.

Biden also repeated that under no circumstances would he send US troops to Ukraine, even to rescue US citizens in case of a Russian invasion.

“That’s a world war. When Americans and Russians start shooting one another, we’re in a very different world,” he said.

His comments came as the US State Department, in a travel note updated on Thursday, advised citizens to leave Ukraine and urged those who remain to exercise caution due to “potential combat operations should Russia take military action”.

“Do not travel to Ukraine due to the increased threats of Russian military action and COVID-19; those in Ukraine should depart now via commercial or private means,” the advisory said.

There are four levels of US warning, the lowest being “exercise normal precautions”.

Ukraine is already at the highest “do not travel” level because of COVID-19 and tensions with Russia. But for US citizens already in Ukraine, the previous advisory said they “should consider departing” from the country.

The new guidelines also warned Americans that the “US government will not be able to evacuate” them in the case of Russian military action anywhere in Ukraine.

“Military action may commence at any time and without warning and would also severely impact the US Embassy’s ability to provide consular services, including assistance to US citizens in departing Ukraine,” it said.

The Russian military has been amassing troops near the country’s border with Ukraine, sparking a diplomatic crisis and heightening fears in the US and Europe that Russia may be preparing for an imminent invasion of its neighbour.

Russia has denied that it is planning to invade Ukraine, but it has vehemently opposed its neighbour’s efforts to join NATO.

Moscow wants security guarantees that the US-led alliance will stop its expansion into former Soviet republics, but Washington and NATO have rejected the demand as a “non-starter” while saying they are open to discussing arms control measures in Europe.

US officials have warned Russia of severe economic consequences if it invades Ukraine, underscoring that a war would be “horrific” for civilians in the region.

Last month, Washington ordered the departure of family members of staff at its embassy in Kyiv, and it also allowed non-essential employees to leave voluntarily.

Early in February, the US military deployed additional troops to Eastern Europe in what it called a signal of Washington’s commitment to the security of NATO members in the region.

But US officials have ruled out a military confrontation with Russia if it decides to carry out an incursion into Ukraine, which is not a member of NATO.