Vladimir Putin has boasted to European leaders that his forces could sweep into Kiev in two weeks if he wanted.
The Russian president reportedly made the threat to the European Commission president during talks on the Ukraine crisis.
Mr Putin told Jose Manuel Barroso: “If I want to, I can take Kiev in two weeks,” Italy’s La Repubblica newspaper reported, implying this could be the result if the EU stepped up sanctions against Russia.
His comments, relayed by Mr Barroso to colleagues at last weekend’s EU summit, emerged as Nato announced it would build a new “spearhead” rapid reaction force of up to 4,000 troops that can be flown into eastern Europe in 48 hours to respond to possible Russian aggression.
The EU’s new head of foreign policy, Federica Mogherini, also warned there was no military solution to what is now Europe’s biggest crisis in decades.
Anders Fogh Rasmussen, secretary general of the alliance, said Nato faced multiple crises on its southern and eastern borders that could erupt at any time.
Leaders of the alliance’s 28 members are expected to agree to the new force at this week’s Nato summit in Wales, and it is likely to include British troops.
The spearhead force is part of a package of measures to sharpen up the alliance as it faces crises in Iraq and Ukraine.
A senior Nato official said allies would take turns to command the spearhead and many of the arrangements would be in place by the end of the year. Troops would be based in their home countries and come together when necessary.
The summit will agree to stockpile supplies in eastern Europe so that equipment and ammunition are waiting for the force when it arrives.