By Idrees Ali and Yeganeh Torbati

WASHINGTON (Reuters) - U.S.-backed forces fighting to drive Islamic State out of northern Syria have gathered a massive trove of documents and data belonging to the militant group, potentially shedding more light on its operations, a U.S. military official said on Wednesday.

The material, gathered as fighters moved from village to village surrounding the town of Manbij, includes notebooks, laptops, USB drives, and even advanced math and science textbooks rewritten with pro-Islamic State word problems, Colonel Chris Garver, the U.S. military spokesman in Iraq, said in a news briefing.

The U.S.-backed fighters - an alliance of Kurdish and Arab forces - have gathered more than 4 terabytes of digital information, and the material, most of it in Arabic, is now being analyzed by the U.S-led coalition fighting the militant group.

"It is a lot of material, it is going to take a lot to go through, then start connecting the dots and trying to figure where we can start dismantling ISIS," Garver said, using an acronym for Islamic State.

Coalition advisers, knowing that Manbij served as a strategic hub for Islamic State, specifically described to fighters the kind of digital and other material to gather as they battled the group's forces, Garver said.

A U.S. special forces raid last year in Syria against a senior Islamic State leader, Abu Sayyaf, produced 7 terabytes of data, U.S. officials said, revealing information about the group's leadership, financing, and security.

The information gathered around Manbij has so far shed light on how Islamic State processes foreign fighters once they enter Syria, Garver said. Manbij served as a key receiving area for foreign fighters on their arrival.

"As a foreign fighter would enter, they would screen them, figure out what languages they speak, assign them a job and then send them down into wherever they were going to go, be it into Syria or Iraq," Garver said.

Fighters from the U.S.-backed alliance have in recent weeks made incremental advances as they try to flush out the remaining Islamic State fighters in Manbij.

(Reporting by Idrees Ali and Yeganeh Torbati; Editing by Jonathan Oatis)