At least 20 Islamic State group fighters were killed Thursday in a wave of strikes by Syrian and Russian warplanes targeting jihadist positions in the city of Palmyra, a monitor said.

The jihadists were killed and more than 50 injured "in at least 35 air strikes targeting areas of the city of Palmyra by Syrian and Russian warplanes", the Syrian Observatory for Human Rights said.

Clashes pitting regime forces against IS jihadists raged nearby, the Britain-based group said.

IS seized Palmyra, a UNESCO World Heritage Site in eastern Syria known as the "Pearl of the Desert", last May, sending shockwaves across the world.

In September, satellite images confirmed that the Temple of Bel, the main temple at Palmyra, had been targeted by IS as part of a campaign to destroy pre-Islamic monuments, tombs and statues it considers idolatrous.

UN experts said the main building of the temple plus a row of columns had been destroyed.

The Syrian army and its Russian ally are reportedly preparing to launch a major operation to retake Palmyra.

Images distributed by the Observatory, which relies on a broad network on the ground, purported to show residential areas of Palmyra in ruins after months of air strikes.

"Dozens of residents have been killed and hundreds of others wounded" in these strikes, the Observatory said.

Russia launched an air campaign in Syria in September that it says is targeting "terrorists", but it has been accused of hitting non-jihadist rebels and civilians -- claims that Moscow denies.

An unprecedented truce in Syria, which came into force on February 27, does not apply to IS or the Al-Qaeda-affiliated Al-Nusra Front.

Though the truce has largely held, defying expectations, some violations across frontlines that should have stayed calm have been reported.

On Thursday, an AFP correspondent in an opposition-held area of battered northern city of Aleppo reported air strikes on the Myassar district near Nayrab military airfield, which rebels captured in February 2013.

The Observatory said a child was killed in the Myassar strikes and two civilians died in shelling of the rebel bastion of Douma near Damascus.