After Tuesday's call, US National Security Adviser Jake Sullivan said the Biden administration was preparing specific robust responses in the weeks ahead if they were required.
The measures included economic sanctions and other actions such as additional troop deployments to Nato allies in the region and defence equipment for Ukraine.
Mr Sullivan refused to be drawn on what the economic measures might be. But he said Nord Stream 2, a new gas pipeline from Russia to Germany, which is not yet in operation, provided "leverage" for the US and its allies.
Russia's military option could take a variety of forms from a large incursion, to a significant invasion of the eastern part of Ukraine. One aim would be to bring the main fighting elements of the Ukrainian army to battle and to inflict such a defeat upon them that the Kyiv government has to rethink its position.
Invading territory amidst a hostile population has significant risks. Ukraine's armed forces have had some Western weaponry and training and are much improved since 2015. However, Russian forces have also improved over recent years. The firepower Russia is building up is impressive. For all the talk about Ukrainian sovereignty Nato cannot and will not come to Ukraine's aid...Of all the military contingencies, Michael Kofman, of the US Centre for Naval Analyses, still thinks that if it happens it will be big. "I think Russia is in the best position since 2014 economically, politically, and militarily to execute such an operation, which is not to say it will happen, but simply to suggest that there are the fewest constraints relative to other periods when it has conducted offensive operations."
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