DAMASCUS, June 8 (Xinhua) -- The U.S. forces on Thursday carried out a live fire exercise at a gas field it controls in Syria's eastern province of Deir al-Zour, a war monitor reported.

The military exercise was conducted in the Koniko gas field in the countryside of Deir al-Zour, involving helicopters and warplanes firing live ammunition and carrying out airstrikes in the vicinity of the field, according to the Syrian Observatory for Human Rights.

The UK-based watchdog group said the move comes as part of the U.S. forces' efforts to fortify its positions in eastern Syria, particularly after Iranian-backed pro-Syrian government fighters had carried out several drone attacks against U.S. positions in that part of Syria.

The current military exercise came in tandem with arrangements by the U.S. forces to fortify their positions inside the gas field, said the observatory.

The Syrian government has long regarded the presence of U.S. troops in Syria as illegal, demanding an immediate withdrawal of the U.S. from Syria and blaming it for the deteriorating living conditions in Syria given the fact that the U.S. and its allied Kurdish groups control key gas and oil fields in eastern Syria.