US-led coalition airstrikes hit Islamic State group targets in the Syrian province of Homs for the first time on Saturday as it launches what Washington called "near continuous" strikes on the militants.

The coalition also began strikes on Islamic State militant targets in the town of Minbej, east of Syria's second city of Aleppo, as well as multiple targets in the jihadist stronghold of Raqqa.

"At least 31 explosions were heard in Raqqa city and its surroundings," the Observatory said.

A US defence official told AFP on Friday that the coalition mission is now launching "near continuous" combat flight operations over Syria.

The Syrian Observatory for Human Rights said the targets in Homs province were far from the front line where forces loyal to President Bashar al-Assad are fighting. Assad troops control the city of Homs, Syria's third-largest metropolitan centre.

"The US-Arab coalition has for the first time struck IS bases in the eastern desert of Homs province," Observatory director Abdel Rahman said, adding that the positions were in the area of al Hammad, east of ancient city Palmyra.

Washington has been attempting not to let Assad's forces exploit its air campaign against the Islamic State group to take the upper hand in the more than three-year-old civil war.

The strikes also hit Islamic State group targets around the town of Tabqa, which houses an air base as well as two training camps that were captured by the jihadists last month, sealing their occupation of the whole of Raqqa province.

The United States and its Arab allies launched air strikes against Islamic State militant and other jihadist positions in northern and eastern Syria on Tuesday.

AFP