A coalition including Syrian Arab groups has regained a swath of territory in northeastern Syria from Islamic State militants, a US military spokesman says.
The fighters, who are from the Syrian Democratic Forces (SDF) and its Syrian Arab Coalition subgroup, regained 255 square kilometres near the town of al-Hawl, US military spokesman Colonel Steve Warren said on Wednesday (local time).
The group "conducted an attack ... driving ISIL back," Warren said by videoconference from Baghdad, using an alternate acronym for IS.
"This is not a large tactical action," he said, but "we are encouraged by what we saw."
The spokesman said the operation had pitted "well over a thousand friendly forces" against "several hundred enemies" in the vicinity, after heavy US airstrikes had cleared the way.
Warren said the United States intended to "reinforce" the action, seeming to hint at further ammunition air drops to US-allied groups after those that took place last month.
The Syrian Democratic Forces were formed in mid-October as an alliance between the powerful Kurdish People's Protection Units (YPG) and other Syrian rebel groups.
The Pentagon's announcement came just a day after The New York Times published an article calling into question the capabilities of the SDF and the Syrian Arab Coalition.
Referring to the SDF, The Times said that "nearly all the group's fighting power comes from ethnic Kurdish militias" -- suggesting it was not quite the coalition of Arabs and Kurds it claimed to be.
Meanwhile, the name "Syrian Arab Coalition," one senior official said, was "an American invention." The group is supposed to have 4,000 to 5,000 fighters.
The US military has been struggling to show substantial results of its support for anti-IS forces.
It has already scrapped a US$500 million (NZ$757.65 million) program to build a rebel army to fight IS, after many candidates failed to pass the screening process and one group gave equipment to an Al-Qaeda affiliate.
After the debacle, the US decided simply to supply equipment and air support to certain rebel groups, particularly in northern Syria.
In mid-October, it parachuted 50 tonnes of small-arms ammunition and rockets to rebels fighting IS.
The White House announced on Friday that US President Barack Obama had authorised the first sustained deployment of special forces to Syria, reversing a long-standing refusal to put US boots on the ground.
The president approved the deployment of "fewer than 50" special operations personnel in the north of the war-ravaged nation, but Warren said no special operations forces were on the ground in the latest operation.
The al-Hawl offensive was supported by 17 air strikes that included A-10s - an anti-tank aircraft - and an AC-130 Spectre gunship, a ground attack plane, from Incirlik, Turkey, Warren said.
In Syria, the SDF and the Syrian Observatory for Human Rights, which seeks to document abuses in that country's civil war, have confirmed the death during the al-Hawl offensive of a Canadian national fighting for IS.
An SDF spokesman said the man died Wednesday during a suicide attack by a jihadist.