The U.S.-led coalition against Islamic State said on Tuesday the fight for the Syrian city of Raqqa would be "long and difficult" but would deliver a "decisive blow to the idea of ISIS as a physical caliphate".

"It’s hard to convince new recruits that ISIS is a winning cause when they just lost their twin 'capitals' in both Iraq and Syria," a coalition statement cited Lt. Gen Steve Townsend, the coalition commanding general, as saying.

"We all saw the heinous attack in Manchester, England," said Townsend. "ISIS threatens all of our nations, not just Iraq and Syria, but in our own homelands as well. This cannot stand," he said.

"Once ISIS is defeated in both Mosul and (Raqqa), 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 said.