Aided by U.S.-led air strikes, the YPG has driven Islamic State from wide areas of northern Syria over the last 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 toward Ain Issa, a town about 60 km 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 this morning 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.
