By Pavel Polityuk and Anton Zverev KYIV/DONETSK, Ukraine (Reuters) -The United States ordered some embassy staff to leave Ukraine on Saturday due to a Russian military buildup, while two sources said Washington was also withdrawing its staff at the Organisation for Security and Cooperation in Europe from Ukraine. Tensions have been mounting for weeks during a Russian military buildup near its ex-Soviet neighbour that has fuelled fears that Russia could attack and on Friday Washington said an invasion could happen anytime. Russia denies such plans. “Today, the @StateDept ordered non-emergency U…