At least 87 people have been killed and dozens more injured in a series of air strikes carried out by the Syrian government on the Islamic State (IS) stronghold of Raqqa.

Rami Abdulrahman from the Syrian Observatory for Human Rights said that 10 Syrian government jets had struck Raqqa at least 10 times, killing dozens.

"The majority of the strikes were in the eastern part of the city," Abdulrahman said. "At least 36 of those killed are civilians. As for the rest, we are not sure yet if they were fighters."

"Most of the casualties were caused by two consecutive air strikes targeting the city's industrial area," he went on to add. "The first strike came, residents rushed to rescue the wounded, and then the second raid took place."

Reuters reported an IS fighter as saying at least 70 people had been killed.

Abu Ibrahim al-Raqqawi from the activist group Raqqa is Being Silently Slaughtered, told the Associated Press “this is one of the ugliest regime massacres in Raqqa to date."

The National Coalition for Syrian Revolutionary and Opposition Forces, a collection of groups opposes to Syrian President Bashar al-Assad released a statement condemning the "heinous crime".

"There are many among the Syrians who now believe that the Assad regime is believed to be the sole benefactor of the US led coalition airstrikes and it is time to review the strategy against ISIL on the ground," said the statement.