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

Two Syria-based groups as well as the Britain-based Syrian Observatory for Human Rights say warplanes carried out at least 12 airstrikes across the city. They reported casualties but had no specific figures.

It was not immediately clear what was hit.

The city in northern Syria is the self-declared capital of the Islamic State group's so-called Islamic caliphate it has established across parts of Syria and Iraq. The region is often targeted by a U.S.-led coalition fighting IS from the air. The Syrians have occasionally targeted the city as well, but Syrian government airstrikes typically concentrate on other opposition-held areas of the country.

Syrian President Bashar Assad reiterated in an interview with Russian media aired Wednesday that that the U.S. refuses to coordinate with his government in the fight against IS.

For the U.S. officials, "if they cooperate with the Syrian army, this is like recognition of our effectiveness in fighting ISIS," Assad said, using another acronym for the group. 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 IS.

Thursday's airstrikes on Raqqa coincided with a visit by the U.N. 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.