The US-led coalition launched 16 airstrikes on Saturday and early Sunday in Raqqa, Syria, which the Islamic State has declared as the de facto "capital" of what it calls its caliphate, a US Army official said, adding that the airstrikes were meant to weaken ISIS' capability to transfer its military hardware.

Drones targeted key ISIS buildings and transit routes in the jihadist group's stronghold, Fox News reported.

"The significant airstrikes tonight were executed to deny Daesh the ability to move military capabilities throughout Syria and into Iraq," said Lt. Col. Thomas Gilleran in a statement, using the derogatory Arabic name for ISIS.

Gilleran, who speaks for the Combined Joint Task Force's Operation Inherent in Resolve, also described the coalition's assault as one of the "largest deliberate engagements" held in Syria to-date.

The overnight airstrikes, which was described by Fox News as being "rare in their intensity," caused successive blasts that shook Raqqa and started a frenzy among the city residents.

"It will have debilitating effects on [ISIS'] ability to move from Raqqa," said the army official.

US Defense Secretary Ash Carter agreed, saying the strikes would limit ISIS' "freedom of movement and ability to counter those capable Kurdish forces," The Guardian reported.

"That's the manner in which effective and lasting defeat of [ISIS] will occur, when there are effective local forces on the ground that we can support and enable so that they can take territory, hold territory and make sure good governance comes in behind it," said Carter in a news conference on Monday with French defence Minister Jean-Yves Le Drian.

A militant website connected with the ISIS confirmed the airstrikes, saying it killed 10 people, including two young boys who were described as civilians. Scores of people were also injured.

A Raqqa-based anti-ISIS activist network, meanwhile, said eight civilians were killed, including a 10-year-old child. The group's report could not be independently verified.

The network, named as Raqqa is Being Silently Slaughtered, said at least one assault targeted a group of ISIS members in the city centre.

Another airstrike targeted an ISIS checkpoint while still another ruined large parts of an ISIS-ran brick factory in the city.