BEIRUT (AP) — A senior official with the main U.S.-backed force in Syria says his fighters have captured the country's largest gas field from the Islamic State group in an eastern province that borders Iraq.

Nasser Haj Mansour of the Syrian Democratic Forces says the Conoco gas field came under full control of the group on Saturday after days of fighting with the extremists.

Britain-based Syrian Observatory for Human Rights, which tracks the war in Syria, says SDF fighters have not yet taken the field in the province of Deir el-Zour, saying fighting is ongoing outside it.

SDF fighters have been marching on the east bank of the Euphrates River in Deir el-Zour while Syrian troops are gaining in areas on the west bank of the river under the cover of Russian airstrikes.