BEIRUT — U.S.-backed fighters captured Syria’s largest oil field from the Islamic State on Sunday, marking a major advance against the extremists in an area coveted by pro-government forces.

With the Islamic State in retreat, the Kurdish-led Syrian Democratic Forces and the Syrian government have been in a race to secure parts of the oil-rich Deir el-Zour province along the border with Iraq.

The Al-Omar oil field was a major source of income for the militant group and is considered one of Syria’s most productive. The condition of the field, which has been controlled by the Islamic State for three years, was not clear following intense coalition and Russian airstrikes.

The SDF, with air support from the U.S.-led coalition, said it captured the field in a “swift and wide military operation.”

It said some militants have taken cover in oil company houses nearby, where clashes are underway. The U.S.-led coalition confirmed the SDF had retaken the oil field.

After coming under heavy fire from the Islamic State, pro-government forces retreated from the area around Al-Omar field, according to the Britain-based Syrian Observatory for Human Rights. The SDF said government forces were 2 miles away from the fields.

Syrian troops, backed by Russian warplanes and Iranian-sponsored militias, have retaken nearly all of the provincial capital of Deir el-Zour, as well as the town of Mayadeen, another Islamic State stronghold, which is across the Euphrates River from the Al-Omar field.

The SDF focused their operations in rural Deir el-Zour on the eastern side of the river, and have already seized a major natural gas field and other smaller oil fields.

The Islamic State captured Al-Omar in 2014, when the group swept across large areas in Syria and neighboring Iraq.