BEIRUT — U.S.-backed forces seized control of Syria's biggest oil field on Sunday, accelerating a race with the Syrian government and its Russian and Iranian allies to take over the last major strongholds of the Islamic State in the east of the country.

Kurds and Arabs fighting under the umbrella of the Syrian Democratic Forces captured the al-Omar oil field in Deir al-Zour province after charging about 60 miles through the desert and launching a surprise assault, according to U.S. military and SDF officials.

The capture of the oil field came two days after the SDF officially declared victory in the Islamic State's de facto capital, Raqqa, freeing up forces for what is expected to be an intensified effort to drive the militants out of their remaining positions in neighboring Deir al-Zour, said Col. Ryan Dillon, a spokesman for the U.S. military.

Deir al-Zour is where most of Syria's oil is located, and it is emerging as a key front in the wider war for influence in the Middle East, between the United States and its allies and the Iranian and Russian alliance that is backing Syrian President Bashar al-Assad.

The capture of al-Omar gives the Kurdish-led SDF control over a vital strategic asset that could serve to give it leverage in any future negotiations over the status of Kurds in Syria and to fund the fledgling autonomous region they are building in northeastern Syria.

It also risks triggering a confrontation with the Syrian government, potentially drawing the United States into a fight with Syria, Russia and Iran.

Syrian government loyalists advancing from the west had reached the outskirts of al-Omar only days before and had seemed poised to take the oil field until the SDF launched its offensive. The oil field contains roughly a quarter of Syria's oil reserves and had been a significant source of income for the Islamic State as it attempted to build a statelike structure in the vast areas it once controlled in Iraq and Syria.

The surprise assault was intended to leave the militant group no time to sabotage the oil field's infrastructure, as it typically does when retreating from important areas, Dillon said. The U.S. military provided support in the form of intelligence and combat advice, he said. But Dillon declined to say whether U.S. Special Operations forces deployed in Syria alongside the SDF had participated in the operation. "We put our forces where they need to be to support our partners," he said.

Al-Omar was taken without "significant damage" to the oil facilities there, according to a statement issued by Liliwe Abdullah, a spokesman for the SDF operation. SDF fighters are now battling with Islamic State holdouts who retreated to a nearby housing complex for workers, the statement added.

Dillon said the U.S. military and its allies are not in a race with the Syrian government to take territory from the rapidly disintegrating forces of the Islamic State. The battle there, nonetheless, is shaping up as a competition between rival powers for control over an area that contains most of Syria's oil reserves and a key regional trade route.

The next major prize is the town of Bukamal, which straddles the highway linking the Iraqi capital of Baghdad to the Syrian capital of Damascus. Control over the border town would enable Iran to reopen a vital land route between Tehran and its Lebanese ally Hezbollah in Beirut, deepening its strategic reach into the heart of the Middle East.

The United States and its SDF allies are also planning to advance toward Bukamal, where the Islamic State has now concentrated its forces, Dillon said. First, however, they will have to clear and consolidate their hold over the large area that was seized Sunday.

"Our mission is to defeat ISIS," Dillon said, using an alternative acronym for the Islamic State. "Our priority was the defeat of ISIS in Raqqa, and now that defeat has happened. They [the SDF] have made this very quick sprint down to al-Omar, and they have to reinforce that area. The offensive operations on Bukamal will be difficult and will take a lot of forces."

Zakaria reported from Istanbul.