The US-led coalition attacking the Islamic State group in Iraq and Syria would struggle to dramatically quicken the campaign, a top Pentagon official said Friday, amid mounting pressure to accelerate the fight.

President-elect Donald Trump has repeatedly said he wants to "bomb the shit" out of IS, and on Thursday his pick for defense secretary, James Mattis, said operations could be intensified -- especially in the push toward the jihadists' stronghold of Raqa in Syria.

But Elissa Slotkin, the acting assistant secretary of defense for international security affairs, said the pace of the Raqa campaign can only go as fast as the coalition-backed local forces on the ground.

Commanders "have a plan that I believe is pushing to the limit what we can do on intensifying that campaign," said Slotkin, a political appointee who will not be working under Trump.

In Syria, the coalition is providing weapons, training and air support to Kurdish and Arab forces as they work to push IS from Syria.

A similar tactic is underway in Iraq, with the coalition backing Kurdish peshmerga and Iraqi security forces as they fight for Mosul.

Slotkin said anti-IS operations in Syria were creating a "snowball effect," with each new victory generating more recruits willing to fight IS. 

She estimated the coalition-backed forces, known as the Syrian Democratic Forces, now numbered about 50,000.

Speaking more broadly of the anti-IS effort, Slotkin said she had a "hard time" seeing how the campaign could proceed much faster, and did not see what additional targets could be attacked that weren't already on the military's radar.

"I would have some questions about what exactly are you striking if you just launched a big campaign," she said.

The coalition has been making steady gains against IS, with the jihadists having lost control of many of the main cities they had held in Iraq in Syria. 

Outgoing Pentagon chief Ashton Carter said in an interview Thursday that IS leader Abu Bakr al-Baghdadi's "days are numbered." 

Source: AFP