DONETSK, (Ukraine): US staff at the Organisation for Security and Cooperation in Europe (OSCE) started to withdraw in cars from the rebel-held city of Donetsk in eastern Ukraine on Sunday, a Reuters witness said, amid fears of a possible Russian invasion.

The OSCE did not respond to a request for comment.

Several armoured cars with the OSCE’s logo were loaded with suitcases and seen leaving the monitoring mission’s headquarters early on Sunday.

The OSCE’s Special Monitoring Mission to Ukraine has been deployed in eastern Ukraine since the outbreak of a war between Ukrainian troops and Russian-backed forces that Kyiv says has killed more than 14,000 people.

The United States and its allies have urged their citizens to leave Ukraine right away to avoid the threat of a Russian invasion, including an air assault, warning 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.

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.

Mykhailo Podolyak, an adviser to the Ukrainian president’s chief of staff, told Reuters that Kyiv had an interest in the Special Monitoring Mission working at full strength, but declined to comment further on what he said was an OSCE matter.

Denmark’s OSCE monitors also left Donetsk, one diplomatic source said. Reuters could not immediately establish whether they were leaving the country or only rebel-held territory.

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.

Out of 680 OSCE monitors in Ukraine, 515 are based in the eastern part of the country, according to the mission’s official website.