The finance minister for the Islamic State of Iraq and Syria (ISIS) and two of the group's other senior leaders were killed in recent U.S.-led coalition airstrikes, a Pentagon spokesman said Thursday.

Army Colonel Steve Warren said that a November airstrike had killed Abu Salah, who he called "one of the most senior and experienced members" of ISIS' financial network.

"Killing him and his predecessors exhausts the knowledge and talent needed to coordinate funding within the organization," Warren said.

The spokesman said U.S. airstrikes also killed Abu Maryam, an ISIS "enforcer and senior leader of their extortion network," and Abu Rahman al-Tunisi, described as an ISIS executive officer who coordinated the transfer of information, people and weapons.

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Warren said the three were killed in airstrikes near the northern Iraqi town of Tal Afar.

"Their removal will degrade ISIL's ability to command and control troops, and it disrupts their ability to finance their efforts," Warren said, using another common acronym for ISIS.

Warren said U.S. airstrikes in recent days killed an estimated 350 ISIS fighters holed up in the western Iraqi city of Ramadi, suggesting the extremists lost as much as half of their defending force.

He estimated there had been 600 to 1,000 ISIS fighters inside Ramadi, which the extremist group captured in May.

Despite this depletion of ISIS forces, U.S. officials are reluctant to predict how long it will take to reclaim the city, which is the capital of Anbar province and a key to the Iraqi government's hopes of restoring its borders.

Defense Secretary Ash Carter, speaking separately Thursday, said Iraqi progress in retaking Ramadi has been "disappointingly slow."

"I am certain it will fall, and we will assist in the making of it fall," Carter said of Ramadi. He added that the U.S. would be willing to commit U.S. Army Apache attack helicopters to the battle for Ramadi, if the Iraqi government requests it and if it would "make a strategically decisive difference."

Asked about that comment later, Warren said, "Apache helicopters are ready," if Washington and Baghdad give the go-ahead.

U.S. officials have frequently expressed frustration with how long it has taken the Iraqi army and other security forces to press an offensive in Ramadi, but in recent days, they have pointed to important battlefield progress.

On Tuesday, Iraqi forces seized an ISIS operations center and the strategic Tamim neighborhood on the outskirts of the city.

With approximately 10,000 troops, including federal police, committed to the fight for Ramadi, the Iraqis vastly outnumber the defenders. Iraqi Maj Gen. Ismail al-Mahlawi, head of military operations in Anbar province, said Thursday there are approximately 300 ISIS fighters trapped in the center of Ramadi, which lies on the Euphrates River.

In other ISIS-related developments Thursday: