DONETSK, Ukraine (Reuters) - Six blasts could be heard on Tuesday in the centre of the separatist-held city of Donetsk in eastern Ukraine, a Reuters reporter said.

       The origin of the explosions was not clear, but a diplomatic source told Reuters earlier that shelling had resumed on the line of contact between government and separatist forces in eastern Ukraine.

       (Reporting by Polina Nikolskaya; writing by Tom Balmforth; Editing by Catherine Evans)