U.S. forces kill an Islamic State leader during a raid in Syria.

WASHINGTON — American Special Operations forces mounted a rare raid into Syria early Saturday, killing a senior leader of the Islamic State and about a dozen militant fighters, as well as capturing his wife and freeing an 18-year old Yazidi woman whom Pentagon officials said had been held as a slave.

In the first successful raids by U.S. ground troops since the military campaign against the Islamic State began last year, two dozen Delta Force commandos entered Syria aboard Black Hawk helicopters and V-22 Ospreys and killed the leader, a man known as Abu Sayyaf. One U.S. military official described him as the Islamic State’s “emir of oil and gas.”

Weeks of surveillance

Even so, Abu Sayyaf is a mid-level leader in the organization — one terror analyst compared him to Al Capone’s accountant — and likely is replaceable in fairly short order. And the operation, while successful, comes as the Islamic State has been advancing in the Iraqi city of Ramadi, demonstrating that the fight against the Sunni militant group in both Iraq and Syria remains very fluid.

Yet the Pentagon’s description of a nighttime raid that found its intended target deep inside Syria without any American troops being wounded or killed illustrates not only the effectiveness of the Delta Force, but of improving U.S. intelligence on shadowy Islam- ic State leaders.

A Defense Department official said Islamic State fighters who defended their building and Abu Sayyaf tried to use women and children as shields, but the Delta Force commandoes “used very precise fire” and “separated the women and children.” The official said the operation involved close “hand-tohand fighting.” (The accounts of the raid came from military and government officials and could not be immediately verified through independent sources.)

The U.S. forces eventually entered the building where they found Abu Sayyaf and his wife, known as Umm Sayyaf, in a room together. His spouse was captured and later moved to a military facility in Iraq, officials said.

The raid came after weeks of surveillance of Abu Sayyaf, using information gleaned from a small but growing network of informants whom the CIA and the Pentagon have painstakingly developed in Syria, as well as satellite imagery, drone reconnaissance and electronic eavesdropping, American officials said. The White House rejected initial reports from the region that attributed the raid to forces of President Bashar Assad of Syria.

In a statement early Saturday, Defense Secretary Ash Carter said the killing of Abu Sayyaf dealt a “significant blow” to the group. The militant leader was said to be involved in the Islamic State’s military operations and helped direct its “illicit oil, gas and financial operations” that raised the funds necessary for the organization to operate. Officials said the raid was approved by President Obama.

Defense Department officials said the Delta Force soldiers carrying out the raid came under fire soon after they landed near a building used by Abu Sayyaf as his residence, in Al Amr, about 20 miles southeast of Deir al Zour, near the oil facilities that he oversaw for the Islamic State.

Wife’s fate unclear

Abu Sayyaf himself “tried to engage” the commandos, a Defense Department official said, and was shot and killed. The commandos took his wife and the Yazidi woman back to the waiting American aircraft.

The commandos were back in Iraq with the two women around dawn local time, officials said. They said the U.S. forces were able to seize communications equipment and other materials from the site, which may prove useful in intelligence assessments.

The Yazidi woman, Carter said, will be reunited with her family as soon as possible. It was unclear on Saturday what would be done with Umm Sayyaf, who, according to Carter’s statement, is suspected of playing an important role in the group’s activities and “may have been complicit in what appears to have been the enslavement” of the Yazidi woman.

The Yazidis are a religious minority persecuted by the Islamic State.