A string of Syrian regime air strikes on the Islamic State group's self-proclaimed capital Raqa on Tuesday killed at least 63 people, more than half of them civilians, a monitor said.

The air strikes were the deadliest by President Bashar al-Assad's air force against Raqa since the Sunni extremist IS seized control of the city last year.

"Among the 63 killed were at least 36 civilians," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.

"There were also 20 unidentified victims who could be civilians or jihadists, as well as the disfigured remains of at least seven other people," he said.

The director of the Britain-based monitoring group said previously that "most of the casualties were caused by two consecutive air strikes" on Raqa's main industrial zone.

"The first strike came, residents rushed to rescue the wounded, and then the second raid took place," Abdel Rahman, whose group relies on a network of sources on the ground in Syria for its information, told AFP.

Amateur video footage distributed by activists in Raqa showed several bloodied bodies laid out on a street near an apparent bombing site, as an ambulance rushed to the scene.

Aid workers in red overalls bearing the Red Crescent symbol could be seen placing the corpses into white body bags.

Activists from the city meanwhile denounced the raids as a "massacre".

The Islamic State organisation emerged in Syria's war in spring 2013.

It took over Raqa, the only provincial capital to fall from government control since the outbreak of a 2011 revolt, and turned it into its bastion.

- Regime strikes 'most feared' -

Most of the city's civil society activists, as well as rebel fighters who expelled Assad's troops, have either been killed, kidnapped or forced to flee for other parts of Syria or neighbouring Turkey.

For many months, Assad's regime only rarely targeted Raqa city, apparently reserving most of its firepower for areas under rebel control.

But late this summer, the government intensified its air strikes against IS positions in northern and eastern Syria.

On September 6, 53 people were killed in air raids on Raqa, among them at least 31 civilians, according to the Observatory.

The US-led military coalition that has been carrying out air strikes against IS in Iraq and Syria has also targeted the jihadist group in Raqa.

Activists say Raqa's residents fear the government's strikes far more than those of the coalition because most of the casualties from the regime's attacks have been civilians.

Strategically located on the river Euphrates near the border with Turkey, Raqa had a pre-war population of about 220,000 but it is now home to 300,000-350,000 people, including many displaced by the conflict, according to the Observatory.

Since the jihadists first started moving into the city, they have been gradually imposing a brutal yet highly-organised system with all the trappings of a state, experts say.

Elsewhere in Syria, IS members stoned to death two men in the eastern province of Deir Ezzor on Tuesday after claiming they were gay, the Observatory said.

And in the central province of Homs the jihadists beheaded a member of the minority Ismaili community, accusing him of "apostasy," said the monitoring group.