U.S. Marine Gen. Robert B. Neller, the commandant of the Marine Corps, and Sgt. Maj. Ronald Green, his top enlisted adviser, visit Marines on post with 1st Battalion, 7th Marines, at Al Asad Air Base, Iraq on Dec. 21, 2015. (Photo by Sgt. Rick Hurtado / Marine Corps)

PARIS – Defense Secretary Ashton B. Carter said Tuesday that the number of U.S. troops deployed to Iraq will likely increase, citing the recent success in taking the city of Ramadi back from the Islamic State and a need to train more Iraqi military and police units as the campaign against the militants expands.

Carter said that the additional U.S. troops would be part of a larger force of military advisers that also could include more troops from countries whose defense ministers he will meet with here Wednesday. Ministers from France, Britain, Australia, Germany, Italy and the Netherlands are gathering in Paris to discuss how the military campaign in Iraq and Syria should evolve.

“With respect to trainers, I expect the number of trainers to increase, and also the variety of training they are giving,” Carter said while flying from Washington to Paris. “I think we’re certainly open to that…. The president has indicated that wherever there is additional opportunity to make a difference according to the strategy, we’d be willing to do that.”

[With Paris trip, Pentagon chief seeks more help against the Islamic State]

The comments came as the U.S.-led military coalition prepares for major operations in which it will attempt to help local forces take back Mosul in Iraq and Raqqa in Syria. The cities serve as de facto capitals for the Islamic State, also known as ISIS and ISIL, and have been identified by Carter as the sites of a “parent tumor” that must be excised, possibly this year.

The secretary would not specify how many additional U.S. advisers could deploy, but said they would help train units needed to hold areas in Iraq as the campaign moves toward and into Mosul. The city of more than 1 million people was seized by the Islamic State in June 2014.

“As territory is retaken from ISIL moving up to and ultimately including Mosul, there are going to need to be not just ground forces that can seize territory, but police forces that can keep security,” said Carter, of the Iraqis.

Carter also spoke about working with certain forces fighting the militants in Syria. While the original effort to train and equip Syrian rebel forces yielded few recruits, the Pentagon has shifted to working with existing groups that want to fight the Islamic State, rather than trying to “create entire new infantry units from scratch,” the secretary said.

Those groups may want to protect their homes, or to retake areas lost to the Islamic State. U.S. involvement with opposition groups will be limited to training a small number of their leaders, rather than teaching basic fighting skills to all, Carter said

“That means that a few key leaders and key individuals are trained to a very high, exquisite level so that they can be enablers for the entire unit,” he said.

In one recent example, Syrian forces backed by U.S. Special Operations troops serving as advisers were able to take back the Tishreen Dam, a strategically important hub whose capture is expected to put pressure on the militants in Raqqa.