Washington (AFP) - The Pentagon hopes to obtain a raft of intelligence on the Islamic State group, including its foreign networks, during its planned offensive to retake Mosul from the fighters, a military spokesman said Thursday.

Many of the 615 additional soldiers Washington is sending to Iraq are intelligence specialists who will help the Iraqis use any intelligence gathered from the northern city as quickly as possible.

"When you free a city like Mosul, you can expect a tremendous lot of intelligence," Colonel John Dorrian, a coalition spokesman, said in a videoconference from Baghdad.

The coalition expects to find computers, hard drives, USB keys and other storage devices abandoned by IS during the fight, as was the case during the recent recapture of the Syrian city of Manbij.

Nearly 20 terabytes of data were collected in the offensive of that city, which had served as a logistical hub for IS near the border with Turkey.

"Information taken in Manbij has been distributed to security services throughout Europe," Dorrian said.

The additional US forces, announced Wednesday, will be deployed to Iraq in the coming weeks, American officials said.

IS seized Mosul along with other areas in June 2014, but Iraqi forces have since regained significant ground from the jihadists and are readying for a drive to retake Iraq's second-largest city.

In addition to intelligence gathering, the US forces will boost the Iraqi military's logistical capabilities, including on the sprawling Al-Asad Airbase in Anbar province.

Some 3,000 to 4,500 IS fighters -- both foreign and Iraqi -- are in the city, according to Dorrian, slightly lower than previous estimates of around 5,000.

"They do keep loosing people because we keep hammering with air strikes," Dorrian said.