New Delhi: With the tension simmering between Moscow and Kyiv, US President Joe Biden has asked its citizens in Ukraine to leave the country immediately and said sending US troops for an evacuation would mean "world war".


The warnings grew stronger with Biden saying "American citizens should leave now."


In an interview with NBC News, he said "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."


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Biden warned about potential major conflict with Moscow in case US and Russian troops engage each other on the ground.


On asked about the scenario where US troops would enter Ukraine to evacuate Americans, Biden said: "There's not. That's a world war when Americans and Russia start shooting at one another."


Meanwhile, the US State Department once again issued a revised advisory urging Americans in Ukraine to leave the country at the earliest possibility, strengthening earlier warnings that urged its citizens to "consider" such 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. If remaining in Ukraine, exercise increased caution due to crime, civil unrest, and potential combat operations should Russia take military action. Some areas have increased risk," the advisory said on Thursday.


On January 23, the state department also facilitated the evacuation of US diplomats' family members and of direct-hire employees. The state department also suggested that US citizens who were present in Ukraine should consider departing right away due to the unpredictable security situation.