Syrian regime air strikes on the Islamic State group stronghold Raqa killed at least 95 people as a government delegation prepared for talks with key ally Russia Wednesday on relaunching peace negotiations.

The bombing on Tuesday was the deadliest by President Bashar al-Assad's air force in Raqa since Sunni extremist IS fighters seized control of the city last year and declared it their capital.

More than half of the dead were civilians, according to the Syrian Observatory for Human Rights, which monitors the war through a network of sources.

It was unknown how many jihadists were killed.

The exiled opposition Syrian National Coalition condemned the strikes as a "brutal massacre," warning that "many seem now convinced that Assad is the major beneficiary of the US-led coalition strikes" against the jihadists.

Raqa was the first provincial capital to fall from regime control, and it was later overrun by IS which has used it as the capital of its self-proclaimed "caliphate" straddling Syria and neighboring Iraq.

The government has in recent months stepped up its air strikes against IS-held towns in the north and east, with most of the casualties reported to have been civilians.

The multi-sided Syrian conflict has killed more than 195,000 people and forced millions from their homes since it began three and a half years ago as an uprising against Assad's regime.

A Syrian regime delegation headed by Foreign Minister Walid Muallem was due to meet Russian President Vladimir Putin at his Black Sea retreat of Sochi on Wednesday.

They were expected to discuss a possible relaunch of peace talks with the opposition, a senior Syrian official said last week.

A second round of UN-brokered talks was held in Switzerland in early 2013 but ended without agreement.

A former leader of the National Coalition, Moaz al-Khatib, is reported to have held talks at the Russian foreign ministry on November 7.

But the coalition has voiced skepticism about prospects for progress. "We are used to the regime trying to put together its handpicked opposition in Damascus, made of up people close to them," its secretary general Nasr al-Hariri told AFP ahead of the talks.