US-led coalition warplanes carried out 30 air strikes on and around the de facto Isis capital in Syria on Friday night.
The strikes targeted positions in the city of Raqqa, in the north-east of the country, and the Division 17 airbase, which was seized by the militants from government troops, according to the Syrian Observatory for Human Rights based in the UK.
Sources in the city reported at least 30 strikes in all to the monitoring group. The Local Coordination Committees, a network of local groups in Syria, also confirmed the strikes.
But neither group had figures for the casualties caused by the warplanes and there has been no immediate confirmation of the strikes by the US Air Force.
Since September of this year, the American led strikes have been targeting Isis strongholds in Syria, as well as in Iraq, where air strikes were already being conducted.
But there are concerns that the strikes are failing to weaken Isis and may even be driving rebels and civilians into their hands.
Syrian foreign minister, Walid al-Moualem, said that the jihadist group would not be tackled unless Turkey was made to tighten its border controls and told a Beirut-based TV programme: "All the indications say that (Isis) today, after two months of coalition air strikes, is not weaker."
The Syrian government has offered to join the western coalition in flighting Isis, but the US refuses to deal with President Assad and maintains that he has lost legitimacy and must leave power.
Since the US-led assault on Raqqa, civilians have died due to air strikes by government warplanes in other northern Syrian towns. The observatory reported that 13 people, including five women and a child, were killed in Jasim following such an attack.
Government warplanes and helicopters also conducted four air strikes on Nawa and one on Tafas, which killed a child.
The monitoring group has further reported that seven people, including two children and two women, have died in the Hatla area owing to aerial attack by government planes.
The strikes come as rebel forces advance north in Syria; in the past two months they have seized a number of towns from President Assad’s forces.
Additional Reporting by AP/Reuters