At least 18 Islamic State leaders have been killed in Iraq and Syria in the past month, a Baghdad-based U.S. military spokesman told reporters at the Pentagon Thursday.

Fox News had reported last week on Special Report that more than a dozen ISIS leaders had been killed in Mosul ahead of the expected ground operation next month in Iraq's second largest city. Some of the ISIS leaders are Chechens who hold a "special place" with the terror group, said Col. John Dorrian, a spokesman for the U.S.-led coalition.

One of the critical tasks facing the Iraqi government in retaking Mosul from ISIS is handling as many as 800,000 refugees expected to pour out of Mosul after ISIS is defeated, Dorrian said. He adds that refugee screening is one of the most important components of the Mosul operation, saying it's a conversation the U.S.-led coalition has with the Iraqis "every day." Dorrian says the screening process must be done under the "command and control" of the Iraqi government.

In June, following the liberation of Fallujah by Iraqi forces, hundreds of Iraqi Sunni refugees escaping the city reportedly were abducted and later killed by Iranian-backed forces outside the city, including Kataaib Hezbollah. The State Department labeled the group a terrorist organization in 2009 for attacking U.S. forces in Iraq.

The United Nations reported in July that 900 Iraqi refugees who left Fallujah were missing and at least 50 had been executed and blamed Iraqi Shia militias, many backed by Iran.

Some of these Iranian-backed forces are now located on the outskirts of Mosul, according to U.S. officials.

Dorrian said the 615 additional troops going to Iraq, an increase that Defense Secretary Ash Carter announced Wednesday, would be the last boost needed to help the Iraqis take Mosul. "We believe this is all the force we will need to liberate Mosul," he said.

Dorrian said a sizable number of the new U.S. troops going to Iraq are intelligence personnel that will be needed to sift through terabytes of information ISIS fighters are expected to leave behind when they either escape the city or are killed.

When the ISIS supply hub of Manbij was liberated near Syria's border with Turkey, U.S.-backed forces recovered 20 terabytes of information, Dorian said. He called it a "treasure trove" of information about ISIS that has since been shared with Western intelligence agencies including those in Europe.

When asked why some troops are going to a remote airbase in western Iraq's Anbar province, Dorrian said the goal is to turn al-Asad airbase located northwest of Ramadi into a 24/7 airport to fly drones and support Iraqi military aircraft. At the moment, the airbase can only support daytime operations, he said.