Syrian regime forces have hammered Raqqa with more than two dozen air strikes, a monitoring group said, ending an undeclared truce between the government and Islamic State group which controls the city.

The Syrian Observatory for Human Rights reported at least 31 fighters from the Islamic State were killed and dozens wounded in the attacks on Sunday. 

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The Observatory said 26 strikes hit Islamic State buildings, including the military court and bases in the city. Al Jazeera cannot independently verify the casualty figures or the targets, although sources told the network that at least six civilians were killed and 10 injured in the bombing.

Government forces have previously held off from targeting the Islamic State group, formerly known as ISIL - a strategy that has aided the group's battle against other rebels such as the Islamic Front coalition, the Free Syrian Army and al-Qaeda's affiliate in the Syrian war, the Nusra Front.

Syria's president, Bashar al-Assad, has long painted the uprising in Syria as a foreign-backed conspiracy and his enemies say he has allowed the Islamic State to grow to promote that idea.

A resident in Raqqa said government jets stepped up their bombardment five days ago. "About 30 percent of the strikes hit the Islamic State positions, the rest hit civilian areas," he said.

On Saturday, there had been 16 air strikes, with dozens of artillery rounds also fired at areas under Islamic State control, he said, speaking via Skype and declining to be identified out of concern for his safety.

The attacks come after the Islamic State group on Thursday captured the headquarters of Syria's 17th Division, based in the Raqqa area. It posted a video online of its operation.

The Observatory says more than 170,000 people have been killed in Syria's civil war. The UN stopped counting at about 100,000 deaths more than a year ago.

Source: Al Jazeera and agencies