US says it will send troops to eastern Europe if Russia invades Ukraine

Biden will also make clear to Putin that the US will not rule out future Ukrainian membership of Nato, as the Russian leader has demanded, a senior US official said.

With an estimated 100,000 Russian troops already gathered within striking distance of the borders, the crisis is at its worst since 2015, when Moscow staged a large-scale incursion into Ukraine, clandestinely sending tanks and artillery to encircle Ukrainian troops and compelling Kyiv to sign a peace agreement in Minsk that has since come close to collapse.

The official pointed out in a briefing to reporters before the Biden-Putin video summit that the first Russian military intervention in Ukraine led to more US troops and equipment to be deployed in eastern Europe, and that there would be similar response this time.

Biden was due to talk to European leaders on Monday in preparation for the Putin summit with the aim of ensuring that he will be presenting a united front. The US secretary of state, Antony Blinken, will also consult the Ukrainian president, Volodymyr Zelenskiy, before the Putin call.

Russia has virtually frozen direct contacts with the Ukraine government and Zelenskiy, whom it has accused of preparing his own attack against regions of south-east Ukraine controlled by Russian-backed forces. Ukraine has strongly denied the claims.

Both western and Russian analysts have said that Moscow is making a credible threat of launching a large-scale military offensive, although there are differing estimates to how likely a Russian offensive is and what may trigger it.

The Russian government has accused Washington of creating war hysteria, but Putin has publicly directed government officials to maintain high levels of tension with the west to make sure that its interests are not ignored.