Jihadists from the Islamic State group were in almost full control of Syria's historic city of Palmyra on Wednesday night after the withdrawal of government forces, a monitor said.

"IS controls almost all of Palmyra" following the withdrawal of government troops from all sectors except for a prison in the east and military intelligence headquarters in the west, said Rami Abdel Rahman of the Syrian Observatory for Human Rights.

IS extremists sparked international outrage earlier this year in Iraq when they blew up the ancient Assyrian city of Nimrud and smashed artefacts in the Mosul museum.