Map locates Omar oil field, where U.S. commandos mounted a raid.

Image Credit: The Associated Press

BEIRUT - U.S. commandos mounted a rare raid into eastern Syria overnight, killing a senior Islamic State leader in a firefight, capturing his wife and rescuing a Yazidi woman held as a slave, the Pentagon said Saturday.

Defence Secretary Ash Carter announced the raid, identifying the militant as Abu Sayyaf. He said no U.S. forces were killed or injured in the operation.

The raid was the first known U.S. ground operation targeting IS militants in Syria. A U.S.-led coalition has been striking the extremists from the air for months, but the only previous time American troops set foot on the ground in Syria was in an unsuccessful commando mission to recover hostages last summer.

The Britain-based Syria Observatory for Human Rights confirmed an attack on the IS-controlled Omar oil field, Syria's largest, saying at least 19 IS members, including 12 foreigners, were killed. The group did not say who carried out the attack, but said it was informed that there was an airdrop that followed the airstrikes. The Observatory relies on a network of activists on the ground in Syria.

Syrian state TV earlier reported that Syrian government forces killed at least 40 IS fighters, including a senior commander in charge of oil fields, in an attack Saturday on the Omar field. The Syrian report, which appeared as an urgent news bar on state TV, was not repeated by the state news agency. State TV didn't repeat the urgent news or elaborate on it.

It was not immediately clear why both Syria and the U.S. would claim a similar operation. The U.S. has said it is not co-operating with President Bashar Assad's government in the battle against the Islamic State group. But it says it usually gives Damascus a heads-up on operations within its borders.

The U.S. did not provide the full name of the militant identified as Abu Sayyaf. There was no information immediately available on jihadist websites.

A statement from the U.S. National Security Council said Abu Sayyaf was a "senior ISIL leader who, among other things, had a senior role in overseeing ISIL's illicit oil and gas operations -- a key source of revenue that enables the terrorist organization to carry out their brutal tactics and oppress thousands of innocent civilians."

"He was also involved with the group's military operations," it said.

A U.S. defence official said the leader killed was a Tunisian national designated by IS as the organization's "emir of oil and gas." The official said Abu Sayyaf had "associations" with Abu Bakr al-Baghdadi, the leader of the Islamic State group, and was thought to have knowledge about a wide range of IS operations.

A senior Iraqi intelligence official said Abu Sayyaf is a former follower of Osama bin Laden who later joined the IS group. The official said Iraqi authorities had given the Americans information about him, saying he was in charge of IS finances. The official spoke on condition of anonymity because he was not authorized to brief reporters.

The U.S. defence official said the raid was conducted overnight Friday (Friday evening Washington time) by a team of Army Delta commandos who flew from Iraq into eastern Syria aboard V-22 Osprey aircraft and Blackhawk helicopters.

Upon arrival at the target, which was a multi-story building, the Americans met stiff resistance. A "fairly intense firefight" ensued, including hand-to-hand combat, said the official, who spoke on condition of anonymity because he was not authorized to discuss details of the raid by name.

The U.S. estimates that about a dozen IS fighters were killed but no civilians were wounded, even though women and children were present. The Americans returned to their base unharmed by about midnight Washington time.

The official said the woman, identified as Umm Sayyaf, is an Iraqi citizen and is in U.S. custody in Iraq.

A senior Obama administration official said Umm Sayyaf was being debriefed to obtain intelligence about IS operations. The official was not authorized to discuss details of the operation by name and spoke on condition of anonymity. The NSC said the wife is suspected of having played an important role in IS operations, including possibly the enslavement of the Yazidi woman.

The National Security Council statement said President Barack Obama authorized the operation upon the "unanimous recommendation" of his national security team.

Asked about the timing of Syrian announcement, National Security Council spokeswoman Bernadette Meehan said the U.S. government did not co-ordinate with the Syrian government or advise it in advance of the operation.

"We have warned the Assad regime not to interfere with our ongoing efforts against ISIL inside of Syria," she said, using another acronym for the IS group. "As we have said before, the Assad regime is not and cannot be a partner in the fight against ISIL. In fact, the brutal actions of the regime have aided and abetted the rise of ISIL and other extremists in Syria."

The statement said the commandos rescued a young Yazidi woman "who appears to have been held as a slave" by the slain militant and his wife. IS militants captured hundreds of members of the Yazidi religious minority in northern Iraq during their rampage across the country last summer.

The extremist group controls much of northern and eastern Syria as well as northern and western Iraq. It holds most of the oil fields in Syria, which are a key source of its funding, and has declared a caliphate governed by a harsh version of Islamic law.

The special operation was announced as IS fighters were advancing in central and northeastern Syria. The militants are nearing the historic city of Palmyra, in the central province of Homs, and have seized a water facility and an oil field on the city's eastern outskirts, activists said.

The advances in the Palmyra countryside were coupled with an announcement Saturday by the IS group that its fighters have seized full control of Saker Island in the Euphrates River near Deir el-Zour, a provincial capital in eastern Syria split between IS and government forces.

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AP National Security Writer Robert Burns in Washington and reporters Darlene Superville in Washington, Maamoun Youssef in Cairo and Qassim Abdul-Zahra in Boston contributed to this report.

News from © The Associated Press, 2015