Syrian militias backed by the US have launched a fresh offensive on Islamic State near its de facto capital of Raqqa.
A US-backed alliance of Syrian militias has launched a new offensive against Islamic State fighters near their de facto capital of Raqqa city, a monitoring group and an official say.
The Syria Democratic Forces (SDF) alliance is the main Syrian partner for the US-led alliance battling the Islamic State group 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 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 towards 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 on Tuesday 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.
New satellite images appear to show extensive damage to a strategic airbase used by Russian forces in central Syria. Moscow has denied reports that an IS attack earlier this month was to blame.
"What the imagery tells us is that first of all this isn't an accidental explosion," Stratfor Analyst Sim Tack said.
"It shows very clearly that there are several different sources of explosions across the airport and it shows that the Russians took quite a bad hit, an entire combat helicopter unit was wiped out."
The images from US intelligence company Stratfor suggest four helicopters and 20 lorries were destroyed in a series of fires inside the base.