A wave of Russian air strikes killed at least 39 civilians on Saturday in Raqa, the main stronghold of the Islamic State jihadist group in Syria, a monitoring group said.

At least five children and seven women were among the dead in IS's de facto capital in the north of the war-ravaged country, the Britain-based Syrian Observatory for Human Rights said.

It said the attacking aircraft were Russian.

Five members of IS's self-styled police force were also killed and 60 people were wounded, some critically, according to the monitor, which relies on a network of sources on the ground.

The air raids came a day after 16 civilians were killed in strikes on the same city.

"What is clear is that their goal is to try to paralyse IS and to stop it from deploying reinforcements from Raqa to the Palmyra area," Observatory director Rami Abdel Rahman told AFP.

IS seized Palmyra, a UNESCO World Heritage Site known as the "Pearl of the Desert", last May.

In September, satellite images confirmed that Palmyra's famed Temple of Bel 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 as well as a row of columns had been destroyed.

In recent weeks, Syrian troops backed by the Russian air force have been pressing an advance to try to reclaim the ancient city.

- 18 IS fighters killed -

On Saturday alone, at least 18 IS fighters were killed in at least 70 strikes on the Palmyra area, the Observatory said, as clashes pitted loyalist troops against jihadists on the ground.

Russia, a key backer of the Syrian regime, on Monday ordered the withdrawal of most of its armed forces from Syria, but continues to strike jihadist targets, particularly around Palmyra.

Senior Russian commander Sergei Rudskoi on Friday said Russian jets were flying around two dozen bombing sorties daily to back up the Syrian government's bid to recapture Palmyra.

"Government troops and patriotic forces with the support of the Russian air force are carrying out a large-scale operation to liberate Palmyra," he told journalists in Moscow.

Roughly 1,800 Syrian civilians including more than 400 children have been killed in Russian air strikes since Moscow launched its aerial campaign on September 30, according to the Observatory.

Moscow has denied claims that its air force has repeatedly hit civilian and non-jihadist rebel targets.

More than 270,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.

An unprecedented ceasefire negotiated by Russia and the United States has largely held since February 27 but the truce does not apply to jihadists.

UN mediator Staffan de Mistura has urged Damascus to make concrete proposals in the coming days on a political transition, following a week of peace talks in Geneva.

But a source close to the regime said Saturday there had been "no progress" at the meetings and criticised the UN envoy for putting pressure on the Syrian government.