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.S. employees at the Embassy to depart due to continued reports of a Russian military build-up on the border with Ukraine, indicating potential for significant military action," the embassy said.
It said that core embassy staff and their Ukrainian colleagues would continue working despite the reduction in diplomatic staff.
The United States on Friday urged its citizens in Ukraine to leave the country right away, saying that it will not send troops to evacuate them if conflict erupts. A string of other countries including Britain, Japan and Australia have also said their citizens should leave.
Russia, which has accused Western nations of spreading lies, meanwhile said on Saturday it had decided to "optimise" its diplomatic staff numbers in Ukraine, fearing "provocations" by Kyiv or another party.
Moscow did not say whether that meant a reduction in staff numbers but said the embassy and consulates in Ukraine continued to perform their key functions.
Separately, two diplomatic sources told Reuters that the United States was pulling out its staff at the OSCE in Ukraine with immediate effect.
The OSCE did not respond to a request for comment.
The OSCE conducts operations in Ukraine including a civilian monitoring mission in the Russian-backed self-proclaimed separatist republics in the country's 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 they expected other nations to make similar evacuation decisions soon.
The 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.
