The US-led coalition "successfully engaged multiple targets" in the northern Syrian city of Raqqa - the de facto capital of the "Islamic State" militant group - on Saturday.

The coalition, comprising more than 60 countries, said in a statement that the 16 air strikes "severely constricted terrorist freedom of movement."

"This was one of the largest deliberate engagements we have conducted to date in Syria and it will have debilitating effects on Daesh's ['Islamic State'] ability to move from Raqqa," spokesperson Lieutenant Colonel Thomas Gilleran said in a statement by the coalition, using the group's Arabic acronym Daesh.

"The significant airstrikes tonight were executed to deny Daesh the ability to move military capabilities throughout Syria and into Iraq," Gilleran added.

The military operation comes a day after the militant group posted a video online showing 25 Syrian soldiers being executed in an ancient amphitheater in the ruins of Palmyra.

The capturing of Palmyra in May sparked outcries from the international community, afraid that the UNESCO World Heritage Site may be the next victim of the group's practice of destroying ancient sites, as it did in Mosul, Nimrud, Hatra and Khorsabad.

The "Islamic State" rose to notoriety in June 2014 when the militant group seized control of Mosul, one of Iraq's largest cities.

ls/bk (AFP, dpa)