The United States has decided to withdraw U.S. staff at the Organization for Security and Cooperation in Europe (OSCE) from Ukraine immediately, two diplomatic sources told Reuters on Saturday, amid fears of a possible Russian invasion. The OSCE did not respond to a request for comment. Tensions have been mounting for weeks during a Russian military buildup near Ukraine and a surge of military activity that has fueled fears that Russia could invade. Russia denies such plans. The OSCE conducts operations in Ukraine including a civilian monitoring mission in the Russian-backed self-proclaimed separatist republics in the countrys east where a war that erupted in 2014 has killed more than 14,000 people. One of the sources, who spoke on condition of anonymity, said the U.S. decision to withdraw its OSCE members was effective immediately and that they expected other nations to make similar decisions soon. Two sources told Reuters that Britain had decided to move its members of the OSCE from the rebel-held regions of Ukraine to the government-controlled area. The United States and its allies have urged their citizens to leave Ukraine right away to avoid an invasion, including a possible air assault, warning an attack could occur at any time. Russia has accused Western nations of spreading lies.