The UK has begun to withdraw staff from the British embassy in Ukraine amid warnings of a Russian invasion.Officials say there have been no specific threats to British diplomats, but about half of the staff working in Kyiv will return to the UK.The US has ordered relatives of its embassy staff to leave, saying an invasion could come "at any time".Russia has denied plans for military action, but tens of thousands of troops have amassed on the border.The embassy moves seem to be precautionary, and nothing specific is thought to have occurred in the past 24 hours to have triggered the decisions of the US and UK.Staff working at the EU embassy will stay in place for now, with EU foreign policy chief Josep Borrell saying he would not "dramatise" the tensions.Members of the Nato alliance, including Denmark, Spain, Bulgaria and the Netherlands, are sending more fighter jets and warships to Eastern Europe to bolster defences in the region.With an estimated 100,000 Russian troops now at the border with Ukraine, the head of Nato has warned there is a risk of fresh conflict in Europe.The decision by the US is one of a number of precautions the state department employs when crises could put American diplomats in harm's way.A travel advisory said the situation was "unpredictable" and that "there are reports Russia is planning significant military action against Ukraine".The US has warned people not to travel to Ukraine and Russia due to the "potential for harassment against US citizens". Non-essential embassy staff have been given permission to leave and US citizens have been told to do the same.The US has stressed it is not an evacuation, but the state department told AFP news agency that if there was a Russian invasion, it would "not be in a position to evacuate US citizens".Ukraine said it was "premature" of the US to withdraw staff and "a display of excessive caution".