The Syrian Democratic Forces (SDF) have since November been fighting to encircle ISIL in the northern Syrian city, which was one of the first cities ISIL seized.

Raqqah has since been used by the terror group to plan and coordinate attacks across the globe, according to U.S. officials.

"We all saw the heinous attack in Manchester, England," U.S.-led coalition commander Lt. Gen. Steve Townsend said in a statement announcing the commencement of the Raqqah operation.

"ISIS threatens all of our nations, not just Iraq and Syria, but in our own homelands as well. This cannot stand," he said, using another acronym for ISIL.

Raqqah is reportedly encircled from the north, west and east following the SDF's advance.

The coalition said the SDF has encouraged civilians to flee Raqqah, and have pledged to hand over control of the city "to a representative body of local civilians who will provide security and governance" after it is liberated.

U.S. support for the SDF has soured relations with Ankara due to the prominent role the PKK/PYD plays in the group.

Turkey considers the PKK/PYD to be the Syrian offshoot of the PKK, a designated terrorist organization in Turkey, the U.S. and EU. However, the U.S. views the PKK/PYD as its ally in the fight against ISIL in northern Syria.

The launch of the Raqqah operation comes as Iraqi forces continue to fight to remove ISIL from Mosul, the terrorist group’s last stronghold in Iraq. 

The dense close-quarters combat in western Mosul has slowed the Iraqi advance, but Townsend said partner forces "are steadily dismantling the physical caliphate of ISIS".

"Once ISIS is defeated in both Mosul and Raqqah, there will still be a lot of hard fighting ahead, but this Coalition is strong and committed to the complete annihilation of ISIS in both Iraq and Syria," he added.