U.S. advisers have joined Iraqi forces mustering in central Iraq for an eventual attempt to retake Mosul, which would be the centerpiece in the campaign to defeat the Islamic State that has now cost the U.S. more than $6 billion.

Citing a Defense Department spokesman, the Hill newspaper reported Wednesday that the overall cost of U.S. military operations against the Islamic State in Iraq and Syria, or ISIS, has climbed to $6.2 billion. Each day, the missions cost an average of $11.5 million, it reported.

The majority of the costs have come from the air campaign that now totals nearly 10,000 airstrikes.

President Obama first ordered U.S. forces to the region in mid-June 2014 as ISIS fighters swept out of Syria into Iraq, and bombing began on Aug. 8, 2014.

The costs were expected to rise as Defense Secretary Ashton Carter looks to "accelerate" the campaign with help from coalition partners to focus on retaking northwestern Mosul in Iraq and Raqaa, the self-proclaimed ISIS capital in northeastern Syria.

The U.S. was expected to deploy more troops in the train, advise and assist role, and also for force protection, in the coming months if the Baghdad government agrees to the proposal, Army Col. Steve Warren, a spokesman for Combined Joint Task Force-Operation Inherent Resolve, said in a video briefing to the Pentagon on Wednesday.

In previous briefings, Warren has said that the additional U.S. troops would number in the "hundreds," and not the thousands. The U.S. currently has about 3,800 troops in Iraq, but the number has fluctuated above 4,000 as troop rotations overlap and some troops are sent on special assignments, according to the Pentagon.

Warren said that some U.S. advisers -- he declined to give a number -- had moved into Makhmour, a Tigris River Valley town about 60 miles southeast of Mosul, to train and assist troops of Iraq's 15th Division who have begun massing in the area for an attack on Mosul that Iraqi Prime Minister Haider al-Abadi has pledged will come this year. The U.S. advisers were backed by additional troops for force protection, Warren said.

"There's a lot going on in Makhmour. That's where one of our operations centers is located. So there are American advise-and-assist capabilities there," Warren said. "That's going to become an area that really kind of directs, I think, the battle going forward" to retake Mosul, he said. Iraqi officials have estimated that about 4,500 troops would be needed for the Mosul offensive.

Makhmour was the site last year of an alleged ISIS mustard gas attack that sickened about 35 Kurdish Peshmerga fighters on the battlefield. Earlier this week, Organization for the Prohibition of Chemical Weapons, a United Nations watchdog agency, said that soil samples from the area had tested positive for sulphur mustard, a banned substance.

Warren said that there were growing signs from U.S. intelligence and open source reporting of dissent in ISIS' ranks and a loosening of its grip on hostage populations in territory in its control.

U.S. and coalition airstrikes on ISIS cash centers and the continuing Operation Tidal Wave II on ISIS oil facilities and delivery trucks "have put a real dent in their wallets," Warren said. He estimated the costs to ISIS in the "hundreds of millions."

On Tuesday, the Associated Press cited Aymenn Jawad al-Tamimi, a researcher with the Middle East Forum and an expert on Islamic State documents, who quoted from an ISIS directive on salary cuts for fighters based in Raqaa.

"On account of the exceptional circumstances the Islamic State is facing, it has been decided to reduce the salaries that are paid to all mujahedeen by half, and it is not allowed for anyone to be exempted from this decision, whatever his position," the directive said.

-- Richard Sisk can be reached at richard.sisk@military.com.

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