Russia was under US pressure yesterday to convince pro-Moscow rebels in Ukraine to cease occupying eastern towns, after Washington warned the situation in the former Soviet republic was in a "pivotal period".
But with the separatists' refusal to budge throwing a deal to defuse the crisis into doubt, and US sanctions looming large, Russia warned that its military was massed on Ukraine's border, ready to act.
In the nearly dozen Ukrainian towns the pro-Kremlin rebels were holding, the stalemate dragged yesterday.
In the major eastern city of Donetsk, gunmen remained barricaded inside the regional government building.
The failure to implement the agreement hammered out in Geneva on Thursday by the US, Russia, Ukraine and the EU threatened to deepen the worst East-West crisis since the Cold War.
The pact calls for the disarmament of "illegal armed groups" and the end to the occupation of seized buildings.
US President Barack Obama has said he wants to see progress within days, otherwise more sanctions would be imposed, on top of those already targeting the inner circle of Russian President Vladimir Putin.
However, Kremlin spokesman Dmitry Peskov said the US threats of more sanctions on Moscow were "absolutely unacceptable".
Meanwhile, Russian President Vladimir Putin welcomed Nato's selection of former Norwegian PM Jens Stoltenberg as its new head, saying the pair had "very good relations" but that it was up to the West to improve ties.
Putin said there was no reason why relations between Russia and the West can not improve, but that it was up to the West to make that happen.
"I think there is nothing that would hinder a normalisation and normal cooperation" with the West, Putin said. "This does not depend only on us. This depends on our partners."