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Donetsk (Ukraine) (AFP) - Russia was under intense US pressure Saturday to convince pro-Moscow rebels in Ukraine to give up eastern towns they are occupying, after Washington said 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 was biting back -- warning that its military is massed on Ukraine's border, ready to act. In the major eastern city of Donetsk, separatist 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.