Russia could invade Ukraine "at any time" and American citizens should leave immediately, the US has warned. An invasion could start with aerial bombing that would make departures difficult and endanger civilians, the White House said on Friday. Moscow has repeatedly denied any plans to invade Ukraine despite massing more than 100,000 troops near the border. The US statement prompted countries around the world to issue fresh warnings to nationals in Ukraine. Belgium became the latest country to urge its citizens to leave the country. The UK, Australia, Canada and the Netherlands are among other nations to issue the same advice. The US is moving its diplomatic staff from the Ukrainian capital Kyiv, according to the Associated Press, which reports that US embassy staff will relocate to the far west of the country near the Polish border. Meanwhile, Russia has decided to "optimise" its diplomatic staff numbers in Ukraine, fearing "provocations" by Kyiv or other party, according to a foreign ministry spokeswoman. However, UK ambassador to Ukraine Melinda Simmons has tweeted that she and a core team are staying in Kyiv. Attempts to de-escalate tensions through diplomacy are set to continue on Saturday, with both US President Joe Biden and France's President Emmanuel Macron due to speak to Russia's Vladimir Putin by phone. Moscow has accused Western countries of stirring up hysteria. US National Security Adviser Jake Sullivan said Russian forces were now "in a position to be able to mount a major military action" in remarks seen as a clear escalation in the urgency of warnings from US officials. "We obviously cannot predict the future, we don't know exactly what is going to happen, but the risk is now high enough and the threat is now immediate enough that [leaving] is prudent," he said. Mr Sullivan added that the administration did not know if Russian President Vladimir Putin had made a final decision to invade, but said that the Kremlin was looking for a pretext to justify military action, which he said could start with intense aerial bombardment. His comments came as US officials warned of a further build-up of Russian troops at Ukraine's borders over the past week and planned Russian military exercises in the Black Sea in the coming days. President Biden has said that he would not send troops to rescue any citizens left stranded in the event of Russian action. On Friday, the US president hosted a video call with transatlantic leaders in which they agreed on co-ordinated action to inflict severe economic consequences on Russia if it invaded Ukraine. The US also said it was deploying a further 3,000 troops from Fort Bragg, North Carolina, to Poland, and that they were expected to arrive there next week. The troops will not fight in Ukraine, but will ensure the defence of US allies.