BEIRUT (AFP) - Syrian regime aircraft Friday carried out at least 25 strikes on Palmyra, killing eight civilians and 12 Islamic militants in the ancient city held by the Islamic State in Iraq and Syria (ISIS) group, a monitor said.

The barrage was "one of the strongest attacks by Syrian warplanes against positions in Palmyra since ISIS captured it" on May 21, said Rami Abdel Rahman of the Syrian Observatory for Human Rights.

Most residents had fled Palmyra since ISIS took over the famed city, leaving only a few of the original inhabitants and the families of militants.

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

Forces loyal to President Bashar al-Assad 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, ISIS' de facto capital of a "caliphate" it has declared straddling Syria and Iraq, killed 18 people, including militants and civilians.

According to the Observatory, 17 people were killed Friday in government air raids on the city of Idlib, stronghold of the Army of Conquest, an alliance of Islamist and militant groups.

Abdel Rahman said the dead included four women.

Elsewhere, the Army of Conquest lost 17 fighters as it intensified its assault on the last two regime-held villages in the northwestern province of Idlib.

Its fighters - including those of Al-Qaeda affiliate Al-Nusra Front - set off at least nine car bombs in the assault, of which at least seven were suicide bombings, said the Observatory.

The blasts targeted positions held by the National Defence Forces, a pro-Assad militia, outside Kafraya and Fuaa, where the Army of Conquest seized three military positions.

The rebel alliance also fired a barrage of more than 370 rockets and shells into the two villages, most of whose residents are Shi'ites.

Since its capture of most of Idlib province, the Army of Conquest has surrounded and heavily shelled Fuaa and Kafraya.

Fuaa, Kafraya and the rebel stronghold of Zabadani in Damascus province were at the centre of two failed attempts last month to secure broad ceasefire deals.