Staff of the Organisation for Security and Cooperation in Europe (OSCE), which has been monitoring the situation in eastern Ukraine, began to pull out of the rebel-held city of Donetsk on Sunday as fears of a possible Russian invasion grew.
A Reuters journalist saw several armoured cars being loaded suitcases and leaving the mission's headquarters.
The OSCE said in a statement that "certain participating states" had told their citizens at the mission to leave within the next days. It did not name the countries but said the mission would carry on with its work.
The OSCE's Special Monitoring Mission to Ukraine has been deployed in eastern Ukraine since 2014 when war broke out Ukrainian troops and Russian-backed rebel. Kyiv says more than 14,000 people have been killed.
Two sources told Reuters that the United States decided to withdraw its staff from Ukraine, while Britain moved its monitors from rebel-held areas to ones under government control.
The United States and others have urged their citizens to leave Ukraine right away to avoid the threat of a Russian invasion, saying an attack could occur at any time.
Russia, which has built up military forces to the north, east and south of Ukraine, has rubbished the idea it plans to attack and has accused Western nations of spreading lies and hysteria.
One diplomatic source said 160 OSCE staff were being taken out of Ukraine, including Dutch, Canadian, Slovakian and Albanian citizens. That number could not be immediately confirmed by another source.
Mykhailo Podolyak, an adviser to the Ukrainian president's chief of staff, declined to comment on what he said was an OSCE matter.
Russia's foreign ministry spokesperson criticised the U.S. move and said the mission was succumbing to what she described as a "military psychosis" stirred up by Washington.
The official, Maria Zakharova, urged the OSCE leadership to prevent attempts to "manipulate the mission" and said its monitoring was needed now more than ever.
Russia and OSCE have had disagreements in the past over eastern Ukraine.
Moscow refused to allow another OSCE mission to keep monitoring the border between the rebel-controlled area in east Ukraine and Russia in September. Pro-Russian separatists blocked its monitors in their hotel in Donetsk for a week in October.
Denmark's OSCE monitors also left Donetsk, one diplomatic source said.
Overall, 21 OSCE monitors left the rebel-held city and more than 30 others also planned to withdraw from nearby government-controlled areas, a diplomatic source said.
(This story refiles to fix spelling error in the headline)
(Reporting by Anton Zverev, additional reporting by Pavel Polityuk in Kyiv; Writing by Vladimir Soldatkin and Maria Tsvetkova; Editing by Tom Balmforth, Kirsten Donovan and Angus MacSwan)
