The State Department on Sunday ordered the families of all American personnel at the U.S. Embassy in Ukraine to leave the country from today amid heightened fears of a Russian invasion.

The department told the dependents of staffers at the U.S. Embassy in Kyiv that they must leave the country. It also said that non-essential embassy staff could leave Ukraine at the government's expense due to 'increased threats of Russian military action'. 

It comes as the UK also started withdrawing diplomats and their families from Ukraine for the same reason. Half of the UK's diplomatic staff and their families stationed at the UK's outpost in Kiev will leave the country. 

But the EU said today that it has no plans to withdraw its own diplomatic staff, with top diplomat Josep Borrell saying there is no need to 'dramatise' the situation.

Meanwhile, NATO allies have put forces on standby and sent ships and fighter jets to bolster Europe's eastern defences, the alliance said Monday.

'NATO will continue to take all necessary measures to protect and defend all Allies, including by reinforcing the eastern part of the Alliance. We will always respond to any deterioration of our security environment,' NATO Secretary General Jens Stoltenberg said in a statement.

The Western alliance pointed to decisions in recent days by Denmark to send a frigate and warplanes to the Baltic states, Spain bolstering naval deployments and the Netherlands putting a 'ship and land-based units on standby' for its rapid response force.

The statement also highlighted a recent offer from France to send troops to Romania and said 'the United States has also made clear that it is considering increasing its military presence'. 

UK Prime Minister Boris Johnson today warned Vladimir Putin that invading Ukraine will be a 'painful, violent and bloody business' as the UK pulled staff from its embassy in Kiev. Mr Johnson delivered the stark message as he admitted that the prospects of averting a confrontation in the area were 'gloomy'.

Tensions are mounting as the West accuses Moscow of threatening to invade Ukraine by massing over 100,000 troops on its border, which Russia denies. NATO's eastern members have been calling for reinforcements. 

Moscow announced new live-fire sea drills that will take place off the coast of Ireland in February. They are part of wider drills involving up to 140 ships across four seas including Pacific and Atlantic.

The Irish government revealed Sunday that it has been warned of drills that will take place within its 'exclusive economic zone' but outside of its territorial waters - around 150 miles off its southwest coast. It said the drills are 'not welcome'.

While massing 100,000 troops on the Ukrainian border, Moscow has also issued a list of security demands to the US and its NATO allies.

The alliance has described most of the list - including a ban on Ukraine joining and the withdrawal of troops from ex-Soviet states - as 'non-starters', though high-level talks are ongoing with the US due to provide written responses this week.