In a rare ground attack deep inside Syria, U.S. Army commandos killed a man described as the Islamic State's head of oil operations, captured his wife and rescued a woman whom American officials said had been enslaved.

A team of Delta Force commandos slipped across the border from Iraq under cover of darkness Saturday aboard Black Hawk helicopters and V-22 Osprey aircraft, according to a U.S. defence official knowledgeable about details of the raid. The official was not authorized to discuss the operation publicly and spoke on condition of anonymity.

The Americans intended to capture a militant identified by U.S. officials as Abu Sayyaf. When they arrived at his location, a multi-storey building, they met stiff resistance, the U.S. official said, and a firefight ensued.

Abu Sayyaf was killed, along with an estimated dozen ISIS fighters, U.S. officials said. There were no American casualties.

Before the sun had risen, the commandos flew back to Iraq where Abu Sayyaf's wife, Umm Sayyaf, was being questioned in U.S. custody, officials said. The goal was to gain intelligence about ISIS operations and any information about hostages, including American citizens, who being held by the group, according to Bernadette Meehan, spokeswoman for the U.S. National Security Council.

'Emir of oil and gas'

Abu Sayyaf was described by one official as the IS "emir of oil and gas," although he also was targeted for his known association with the group's leader, Abu Bakr al-Baghdadi.

ISIS controls much of northern and eastern Syria as well as northern and western Iraq, despite months of American and coalition airstrikes. (The Associated Press) U.S. officials said it was likely, given Abu Sayyaf's position, that he knew about more than just the financial side of the group's operations.

Despite the U.S. claims, much about the ISIS figure was in question. The name Abu Sayyaf has rarely been mentioned in Western reports about the extremist group and he is not known to be among terrorists for whom the U.S. has offered a bounty. The name was not known to counter-terrorism officials who study ISIS and does not appear in reports compiled by think tanks and others examining the group's hierarchy.

The U.S. official said Abu Sayyaf's death probably has temporarily halted ISIS oil-revenue operations, critical to the group's ability to carry out military operations in Syria and Iraq and to govern the population centers it controls.

But U.S. Rep. Adam Schiff, the top Democrat on the House Intelligence Committee, cautioned against exaggerating the long-term gain from killing Abu Sayyaf. He said ISIS, like al-Qaeda, "has proven adept at replacing its commanders and we will need to keep up the pressure on its leadership and financing."

ISIS well-funded

A U.S. Treasury official told Congress in October that ISiS militants were earning about $1 million a day from black market oil sales alone, and getting several million dollars a month from wealthy donors, extortion rackets and other criminal activities, such as robbing banks. Kidnappings were another large source of cash.

U.S. airstrikes in Syria since September have frequently targeted ISIS oil-collection facilities in an effort to undermine the group's finances.

ISIS controls much of northern and eastern Syria as well as northern and western Iraq, despite months of American and coalition airstrikes and efforts by the U.S.-backed Iraqi army to retake territory. ISIS holds most of the oil fields in Syria and has declared a caliphate governed by a harsh version of Islamic law.

Also Saturday, activists said ISIS fighters pushed into the Syrian town of Palmyra, home to famed 2,000-year-old ruins.

The U.S. Army raid occurred one day after the U.S.-led campaign to roll back ISIS gains in Iraq suffered a significant setback in Ramadi, the capital of Anbar province. ISIS fighters are reported to have captured a key government building in Ramadi and have established control over a substantial portion of the city, officials have said.

U.S. House Speaker John Boehner, in a written statement Saturday praising the raid into Syria, said he was "gravely concerned" by the ISIS assault on Ramadi and that it threatened the stability and sovereignty of Iraq.

ISIS has made major inroads at Iraq's Beiji oil refinery complex in recent days. Reports vary, but U.S. officials have said ISIS is largely in control of the refinery, as well as the nearby town of Beiji, the main ISIS stronghold in northern Iraq, which is on the main Baghdad-Mosul route.

Yazidi woman going home

U.S. Defence Secretary Ash Carter in Washington announced the raid, followed soon after by word from the White House.

Meehan, the NSC spokeswoman, said in a statement that the freed woman, a Yazidi, "appears to have been held as a slave" by Abu Sayyaf and his wife. She said the U.S. intends to return her to her family.

IS militants captured hundreds of members of the Yazidi religious minority in northern Iraq during their rampage across the country last summer.

A senior Obama administration official said Umm Sayyaf was being debriefed at an undisclosed location in Iraq 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.

Rare raid

The raid was the first known U.S. ground operation targeting ISIS 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.

U.S. officials said the U.S. did not co-ordinate its operation with the Syrian government.

"We have warned the Assad regime not to interfere with our ongoing efforts against ISIL inside of Syria," Meehan said, using another acronym for ISIS. "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."

An NSC statement said President Barack Obama authorized the raid upon the "unanimous recommendation" of his national security team.

Saturday's raid came as ISIS fighters advanced in central and northeastern Syria. Activists said ISIS fighters pushed into Palmyra, home to famed 2,000-year-old ruins, after seizing an oil field and taking control of the water company on the outskirts.

ISIS said its fighters took full control of Saker Island in the Euphrates River near Deir el-Zour, a provincial capital in eastern Syria split between ISIS and government forces.