United States advisers move closer to the front lines in fight for Mosul

Referring to Islamic State with an Arabic acronym, he said forces were now working to "liberate the people of Mosul from Daesh oppression forever".

So far, Iraqi forces have taken almost 50,000 square miles of territory in the offensive that is being spearheaded south of west Mosul, Iraq's Federal Police said.

The battle for western Mosul promises to be the most daunting yet, as the half of the city west of the Tigris River has older, narrower streets and is still home to hundreds of thousands of civilians, who have been told to shelter in place.

Iraqi special operations forces, regular army and federal police units are taking part in the offensive along with government-approved paramilitary forces, mainly consisting of Shiite militias, which are operating on the city's outskirts.

Around half a million civilians stayed on in east Mosul, making the screening process that would have been necessary to prevent IS members blending in with the rest of the population nearly impossible.

Late in January, Abadi declared the liberation of the eastern side of Mosul, or the left bank of Tigris, after more than 100 days of fighting against IS militants.

It also fears that a protracted siege of holdout jihadists in west Mosul could leave an estimated 750,000 civilians facing starvation there.

Andrew Exum, a senior Pentagon Middle East policy official during the final years of the Obama administration, said it was more likely that conventional USA forces would aid an impending Kurdish-led assault on the city than that the U.S. would restructure its approach to the war wholesale.

The battle for Mosul has also led American troops to come closer to combat situations even though they are still required to be at Iraqi unit headquarters beyond enemy lines.

More news: Trump to pick Alexander Acosta as new nominee for labor secretary

The operation involves Iraqi army troops and militarized police, backed by air power provided by a USA -led coalition.

Mr Mattis declined to go into further detail, saying he owed "confidentiality" to the troops.

About 160,000 civilians have been displaced since the start of the offensive in October, U.N. officials say.

Right now, ISIS fighters in Mosul are using quad-copters that are no larger than a couple feet in diameter and can fly for about an hour, he said.

Islamic State has overrun Mosul - along vast swathes of territory in northern and western Iraq - in 2014.

The involvement of many local and foreign players with diverging interests in the war heightens the risk that they could clash between themselves after Islamic State is defeated.

Part of efforts to defeat the terrorists is to "take every opportunity that we can to remove ISIS leadership figures from the battlefield", he said.

The Rapid Response force is advancing alongside the federal police and captured several villages, according to an officer, who said they had been largely abandoned by IS.

Mattis rejected a suggestion by President Trump that the United States might take Iraq's oil.

  • Adam Floyd