Syrian government helicopters and warplanes carried out a series of airstrikes overnight on a northern town controlled by the Islamic State group, killing at least 21 people, activists said Sunday.

The air raids struck the town of al-Bab in Aleppo province late Saturday and lasted through early Sunday morning. The Aleppo Media Center activist collective and the Britain-based Syrian Observatory for Human Rights both reported the attacks.

Observatory director Rami Abdurrahman said there were 10 strikes in total, including seven so-called barrel bombs dropped from helicopters. He said at least 21 people were killed and more than 100 wounded.

The Aleppo Media Center put the death toll at 30, with 85 wounded. Differences in casualty figures are common in the chaotic aftermath of attacks in Syria.

President Bashar Assad's air force routinely bombs towns held by the Islamic State group, as well as areas controlled by mainstream rebel groups.

A U.S.-led coalition also is conducting an aerial campaign against the Islamic State group and other extremists in Syria. Washington says it does not coordinate its airstrikes with Damascus.

Much of the multination's coalition has recently focused its firepower on Islamic State fighters attacking the predominantly Kurdish town of Kobani on Syria's northern border with Turkey. The extremists launched an offensive against the town, which is also known as Ayn Arab, in mid-September. After making initial gains in Kobani, the Islamic State group's assault has slowed to a bitter grind.

Abdurrahman, the director of the Observatory, said Sunday that more than 1,000 people have been killed in the battle of Kobani, including 609 Islamic State militants. The toll also includes 363 fighters from the Kurdish militia known as the YPG, as well as 24 civilians and more than a dozen Syrian rebels.

In southern Syria, meanwhile, Islamic rebel brigades and the Al Qaeda-linked Nusra Front seized the town of Nawa from forces loyal to Assad, the Observatory said.

"Nusra and the rebels control the whole town and the area around it," Abdurrahman said. "The Syrian regime pulled out because they are weak there."

Activists say more than 200,000 people have been killed in Syria since protests against Assad spiraled into violence in 2011.

United Nations envoy to Syria, Staffan de Mistura, met with Syrian Foreign Minister Walid al-Moallem on Sunday in Damascus, the SANA state news agency reported. It said the two men discussed several issues, including the envoy's recent initiative on establishing local cease-fires as a way to try to halt the fighting in the country.

A Syrian state newspaper on Saturday criticized de Mistura for pursuing his idea of local truces, saying he deviated from the "limits of the international mission" with which he was entrusted.