Syrian government warplanes on Wednesday struck an eastern city that is a stronghold of an Islamic militant group, killing at least 12 people, opposition activists said.

The airstrikes on the city of Raqqa that has been under control of the Islamic State of Iraq and Syria for over a year appear to be part of an intensified Syrian government campaign against the Al-Qaida breakaway group that has in recent weeks become a major fighting force in neighboring Iraq.

The Britain-based Syrian Observatory for Human Rights said the jets struck targets around the provincial capital of Raqqa, including a market and the headquarters of the Islamic State.

Another activist group, the Syria-based Local Coordination Committees also reported the strikes, saying that five people were killed in a single strike that targeted the Islamic State building.

An opposition activist in Raqqa reported seven airstrikes on Wednesday on the city of about 500,000 people. The activist, who goes by the name of Abu Noor, said in an interview over Skype that 13 bodies of civilians have been identified. He said the death toll will likely rise due to a large number of wounded in various attacks around the city.

The activists said only one of the airstrikes hit the Islamic State headquarters in the city, while others targeted other areas, including a market.

The Islamic State, along with other rebel groups fighting to overthrow Syrian President Bashar Assad, captured Raqqa in March 2013.

In neighboring Iraq, Islamic state fighters have in recent days captured major cities and towns in the north and also border crossings with Syria and Jordan from the Shiite-led government in Baghdad.

Also Wednesday, Syrian aircraft carried out a series of air strikes in the eastern, oil-rich province of Deir el-Zour near the border with Iraq, the Observatory said. Islamic state fighters have been battling rival jihadi rebel groups for months in the oil-rich province. There were no immediate reports of reports of casualties.

In Deir el-Zour, at least two airstrikes struck Muhassan, a town along the Euphrates River near Iraq that has been daily pounded by warplanes since the Islamic State fighters captured it last week.