AMMAN (Reuters) - A U.S.-backed alliance of Syrian militias launched a new offensive against Islamic State fighters near their de facto capital of Raqqa city on Tuesday, a monitoring group and an official said.

A member loyal to Islamic State waves a flag in Raqqa June 29, 2014. REUTERS/Stringer/File Photo

The Syria Democratic Forces (SDF) alliance is the main Syrian partner for the U.S.-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 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 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.

The ground operation was accompanied by a series of air strikes on militant hideouts in the area carried out by jets belonging to a U.S.-led coalition.

The attack follows a recent visit to northern Syria by U.S. Central Command Commander General Joseph L. Votel , the highest-ranking U.S. military official to visit Syria since the war erupted in 2011.

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.

Raqqa city sits in the centre of the province of the same name, which borders Turkey to the north.

During his visit, Votel met some SDF commanders as well as U.S. troops who are training the force, the U.S. Defence Department said.

The SDF have driven Islamic State from wide areas of northern Syria and in February captured the town of al Shadadi, a major logistics hub for the militants that was connected by a network of highways in Hasaka province. Its capture had further isolated Raqqa.

Islamic State’s territory in Iraq and Syria has shrunk significantly from its peak. The group is also being targeted in a separate campaign by the Syrian military and its allies, including Russia.