Syrian regime aircraft Friday carried out at least 25 strikes on jihadist-controlled Palmyra, the country's famed ancient city, with eight civilians among the dead, a monitoring group said.

The Syrian Observatory for Human Rights said the raids killed "at least eight civilians and many members" of the Islamic State jihadist group.

The aerial barrage was "one of the strongest attacks by Syrian warplanes against positions in Palmyra since IS captured it" on May 21, according to Observatory head Rami Abdel Rahman.

He said most residents had fled in the wake of the IS takeover, leaving only a few of the original inhabitants and the families of militants.

The jihadists have blown up the ancient Temple of Bel and several famed tower tombs in Palmyra, a UNESCO-listed world heritage site.

Regime forces have fought hard to edge closer to the historic town.

"In the past two days, the regime has intensified its air raids against areas controlled by the Islamic State group," Abdel Rahman said.

On Thursday, government air strikes on Raqa city, IS's de facto capital of a "caliphate" it has declared straddling Syria and Iraq, killed 18 people, including jihadists and civilians.