DONETSK, Ukraine (Reuters) - Multiple explosions were heard on Sunday in the centre of the separatist-held city of Donetsk in eastern Ukraine, a Reuters witness said.

       A loudspeaker in the area asked citizens to exercise caution. The origin of the explosions was not clear.

       (Reporting by Polina Nikolskaya; writing by Polina Devitt; editing by Mark Trevelyan)