BEIRUT — Syrian warplanes carried out a wave of airstrikes in the Islamic State-held city of Raqqa, activists said, in rare attacks that coincided with a visit by a UN envoy to Damascus on Thursday.

In the country’s south, helicopters dropped barrel bombs that struck a busy marketplace, killing at least 17 people. An activist in the rebel-held Bosra Sham said the explosives were dropped as the market was packed with shoppers and people buying necessities for children who go back to school this week.

Speaking on condition of anonymity for security reasons, he said around 24 people were killed and many others wounded, with many of them in critical condition. The Britain-based Syrian Observatory for Human Rights put the death toll at 17. Such discrepancies are common in the aftermath of large bombings.

Two Syria-based groups as well as the Observatory said earlier Thursday that warplanes carried out at least 12 airstrikes across the ISIS-held city of Raqqa in northern Syria. They reported casualties but had no specific figures. It was not immediately clear what was hit.

The city is the self-declared capital of the Islamic State group’s self-styled caliphate, which includes large parts of Syria and Iraq. The region is often targeted by a US-led coalition fighting ISIS from the air. The Syrians have occasionally targeted the city as well, but Syrian government airstrikes typically concentrate on opposition-held areas of the country.

Syrian President Bashar al-Assad reiterated in an interview with Russian media aired Wednesday that the US refuses to coordinate with his government in the fight against ISIS.

For the US officials, “if they cooperate with the Syrian army, this is like recognition of our effectiveness in fighting ISIS,” Assad said. He also said that his priority is “defeating terrorism” in Syria.

Russia is trying to convince the West of the need to work with Syria in the fight against ISIS.

Thursday’s airstrikes on Raqqa coincided with a visit by the UN envoy to Syria, Staffan de Mistura, to Damascus. De Mistura met with Foreign Minister Walid al-Moallem, who repeated to him that fighting terrorism was the Syrian government’s priority, according to the state-run news agency SANA.

More than 250,000 people have been killed in Syria’s civil war, according to UN officials.