American special operations forces killed a senior Islamic State leader in a raid in Syria, US officials said on Saturday, an operation that marked a departure from Washington's strategy of relying primarily on air strikes to target militants there.

Delta Force commandos used UH-60 Black Hawk helicopters and V-22 Osprey tilt rotor aircraft to punch deep into eastern Syria from Iraq. They engaged in a firefight and hand-to-hand combat with Islamic State fighters, killed a key figure in the group named as Abu Sayyaf and captured his wife, US officials said.

The officials described Sayyaf, a Tunisian, as an Islamic State commander who helped manage the group's black-market sales of oil and gas to raise funds. The officials said President Barack Obama ordered the overnight operation, which killed about a dozen Islamic State fighters and had been planned for several weeks.

It marked the first known US special forces operation inside Syria apart from a failed mission last year to rescue US and other foreign hostages held by Islamic State in northeastern Syria.

Wary of the United States getting pulled deeper into Middle East conflicts, Obama has promised not to commit major ground forces in the fight against Islamic State, which has seized swathes of Syria and Iraq. But he has left open the prospect of special forces raids.

It was not immediately clear if this one marked the start of a new chapter in Syria.

US and Arab forces have carried out almost daily air raids against hardline Islamist militant groups in Syria including Islamic State since last September, and US-led forces are also targeting the group in Iraq.

Defense Secretary Ash Carter said the operation was intended to capture Abu Sayyaf but he was killed "when he engaged US forces." His wife, Umm Sayyaf, was captured, then placed in US military detention in Iraq and was being questioned about Islamic State operations and hostages held by the group.

No US forces were killed or wounded during the operation, Carter said.

"The operation represents another significant blow to ISIL, and it is a reminder that the United States will never waver in denying safe haven to terrorists who threaten our citizens, and those of our friends and allies," Carter said, using an acronym for the Islamic State organization.

Local Syrian sources contacted by Reuters said two other senior Islamic State operatives, a Syrian and a Saudi, were also killed alongside Abu Sayyaf. The raid lasted no more than half an hour, according to the sources.

The accounts could not be independently verified. US officials had no immediate comment on whether other senior Islamic State militants were killed in the raid.

The raid in Syria came at a time when Islamic State, which has declared a caliphate in areas it controls and has carried out beheadings and massacres, has scored high-profile gains in Iraq and made advances in Syria.

Islamic State militants raised their black flag over the local government headquarters in the Iraqi city of Ramadi on Friday and claimed victory after overrunning most of the provincial capital.

If Ramadi were to fall it would be the first major city seized by the Sunni insurgents in Iraq since security forces and paramilitary groups began pushing them back last year.

US House of Representatives Speaker John Boehner said he extended his "gratitude and continued support" for the US troops involved in the Syria raid. But Boehner, a Republican, said he was "gravely concerned" by Islamic State's advances in Ramadi, which he said "threatens the stability and sovereignty of Iraq, which is vital to America's interests."

Hawkish Republican critics of Obama, a Democrat, say he has not acted forcefully enough to rein in the rise of Islamic State.

GREEN LIGHT FROM OBAMA

White House National Security Council spokeswoman Bernadette Meehan said the Syria raid was conducted "with the full consent of Iraqi authorities." She said the United States did not give any advance warning or coordinate with the government of Syrian President Bashar al-Assad, who Washington opposes.

"The president authorized this operation upon the unanimous recommendation of his national security team and as soon as we had developed sufficient intelligence and were confident the mission could be carried out successfully," Meehan said.

Meehan said US forces freed a young Yezidi woman "who appears to have been held as a slave" by Abu Sayyaf and his wife. His wife was suspected of playing "an important role in ISIL's terrorist activities," Meehan said.

The US military last summer carried out a failed mission in Syria to rescue journalist James Foley, held hostage by Islamic State. Foley was later beheaded by the group in August 2014.

In December, al Qaeda militants shot and fatally wounded American photo journalist Luke Somers and South African teacher Pierre Korkie during a failed rescue attempt led by US commandos in Yemen.

Saturday's raid followed a summit Obama held at Camp David earlier this week with Saudi Arabia and other Gulf Arab allies who have pressed the United States to be more militarily assertive in Syria, especially in support of moderate rebels seeking to oust Assad.