Smoke rises from Islamic State positions after an airstrike by coalition forces in Mosul, Iraq, on Oct. 18. (Associated Press)

PARIS — As thousands of Iraqi troops slowly encircle the city of Mosul, Defense Secretary Ashton B. Carter met here Tuesday with key coalition members, suggesting that the battle for the Islamic State’s de facto capital in northern Syria could soon begin.

Carter spoke alongside French Defense Minister Jean-Yves Le Drian, discussing what will come after the last Islamic State fighters are purged from Mosul and the coalition sets its sights on Raqqa, Syria. Carter said the U.S.-led coalition is helping to build the local force, including a contingent of Arab fighters, that will be used to enter Raqqa. The volunteers willing to participate could soon find themselves in a protracted and violent urban battle that will, more than likely, serve as the Islamic State’s last stand before it morphs into an insurgency.

A U.S. military official, speaking on the condition of anonymity to discuss future operations, said the U.S.-led coalition hopes to expand the current contingent of U.S.-backed fighters in Syria by a “couple hundred” before the force goes into Raqqa, hopefully using past military successes as a groundwork for their recruitment.

“There will be overlap,” Carter said of the current campaign in Mosul and the future campaign in Raqqa, adding that both operations are proceeding on schedule.

Earlier in the year, U.S. military officials suggested that the campaigns to retake Mosul and Raqqa could happen simultaneously. But with Mosul three times the size of Raqqa, Iraqi forces need to make considerable gains in Mosul before the coalition can muster enough resources to support both operations, the military official said.

[Tough resistance from Islamic State slows Iraqi advance on Mosul]

Officials also discussed Tuesday what form the Islamic State will take if it is defeated in Syria and Iraq. Carter said that the coalition must be “ready for anything” and that he has directed the U.S. Joint Special Operations Command, or JSOC, to combat the Islamic State’s current and future attacks, known as “external operations,” against the West.

Carter added that JSOC is focusing on “destroying” attack plotters as well as their networks and finances.

While the fight for Raqqa could be months away, more than 30,000 Iraqi troops under various flags are inching toward the center of Mosul in the campaign that began this month to retake the Islamic State stronghold in northern Iraq. Fighting around the city — which fell in 2014 to the Islamic State — has been limited to the surrounding suburbs as advancing forces have moved across open desert and into clusters of villages that form an outer ring around Mosul.

On Saturday, U.S. military officials said Kurdish peshmerga fighters had reached the limit of their advance, and Iraqi military units would continue pushing into Mosul. The Islamic State has not tried to hold ground outside the city, instead relying on suicide bombers, roadside bombs and indirect fire to inflict piecemeal casualties on the Iraqi forces before falling back toward Mosul proper.

While Kurdish and Iraqi forces have taken the brunt of the casualties, U.S. troops assisting the Iraqis on the front are also in harm’s way. On Thursday, U.S. Navy Chief Petty Officer Jason Finan, 34, a bomb-disposal technician, was working alongside a Navy SEAL team when he was killed by a roadside bomb. Finan and his team were advising the Iraqi Counterterrorism Service.

As the battle progresses and the Islamic State’s resistance increases, the mix of U.S. troops, artillery and coalition air power will become even more critical for the battle’s success. U.S. military officials have said that air power near Mosul has been increased tenfold and that the campaign is the No. 1 priority for coalition aircraft. As the forces converge from multiple directions, airspace over the city will become increasingly cluttered.

With the liberation of Mosul as the primary focus of operations in Iraq, Islamic State fighters have attempted two other attacks since the campaign began. The attacks, probably designed to pull resources away from the Mosul front, were focused on the northeastern city of Kirkuk and the western town of Rutbah. In Kirkuk, Islamic State fighters killed dozens of Kurdish security forces before being driven out of the town, while in Rutbah, according to local news reports, clashes are ongoing as the militant group has expanded its control during the past day.

Carter said Tuesday that he was “encouraged” by the Mosul campaign’s progress despite the group’s continued resilience and its willingness to fight even though it is on the receiving end of almost every type of aircraft in the U.S. military. He subsequently disclosed the number of Islamic State leaders the U.S.-led coalition had supposedly targeted and killed in Mosul as a metric of its success. Carter said that of the 35 leaders, most were killed in the past 90 days. Though U.S. forces have added air power over Mosul since the battle started a little more than a week ago, U.S. and coalition airstrikes have regularly targeted the city since the air campaign started more than two years ago.

“In fact, you might say the most dangerous job in Iraq right now is to be the military emir of Mosul,” Carter said.

The strategy of targeting terrorist leadership, known in the U.S. military as “High Value Targets” or “High Value Individuals,” has been a staple of how the United States has waged its wars following the Sept. 11, 2001, terrorist attacks. It is unclear how effective the targeted killings are, however, as the militant groups often quickly replace their fallen leaders.

Lt. Gen. Stephen Townsend, commander of the U.S.-led coalition fighting the Islamic State in Syria and Iraq, told reporters Sunday during a news conference in Iraq that U.S. Special Operations forces, combined with airstrikes, have killed numerous midlevel Islamic State leaders in Iraq and Syria. He added that the strikes have probably affected the group’s ability to launch terrorist attacks against the West.

“I think they’re going to pay off in the coming weeks ahead,” Townsend said of the strikes.

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