An American-backed alliance of Syrian militias launched an offensive against Isis (Islamic State) near their de facto capital of Raqqa city, according to a monitoring group and an official.
The Syria Democratic Forces (SDF) alliance is the main Syrian partner for the alliance battling Isis that controls large areas of northern and eastern Syria. Its most powerful component is the Kurdish YPG militia.
Aided by US-led air strikes, the YPG has driven Isis from wide areas of northern Syria over the past year or more, though its advances have recently slowed.
There has been no indication of when a full assault on Raqqa city might take place. A Kurdish official contacted by Reuters declined to say whether it was a target of the latest offensive.
Syrian Kurdish groups have previously said an attack on the predominantly Arab city of Raqqa should be led by Arab militias. Syria experts say the SDF's Arab groups are not yet ready for such an attack, however.
An unspecified number of SDF fighters were seen moving south from their stronghold of Tel Abyad near the Turkish border towards Ain Issa, a town about 60km north west of Raqqa city, and clashes were reported nearby, the Syrian Observatory for Human Rights monitoring group said.
An SDF spokesman, Talal Silo, confirmed a military operation began today but gave no details. He told Reuters via internet messaging it was focused at this stage on capturing large tracts of territory north of Raqqa, not the city itself.
The ground operation was accompanied by a series of air strikes on militant hideouts in the area carried out by jets belonging to a US-led coalition.
The attack follows a recent visit to northern Syria by General Joseph Votel, head of US Central Command and the highest-ranking US military official to visit Syria since the war erupted in 2011.
After meeting commanders of the SDF, Votel said the local forces being trained by American special forces were proving to be "capable and competent partners".
"They're exhibiting their initiative, their innovativeness, their skills their expertise to really make a difference here," he was quoted as saying by the Pentagon.
His visit fuelled expectations of an imminent attack but neither Washington nor its allies have indicated they are about to embark on a campaign to retake Raqqa or Mosul city, the other main stronghold of the militants in Iraq.
Washington's strategy in Syria has shifted from trying to train thousands of fighters outside the country to supplying groups headed by US-vetted commanders.