By Tom Ramstack

FORT MEADE, Maryland (Reuters) - U.S. Army prosecutors presented their last witnesses on Friday in the sentencing phase of Private First Class Bradley Manning's court-martial for providing a trove of secret files to WikiLeaks.

Defense lawyers for Manning, who faces up to 90 years in prison, are expected next week to start laying out their case that the former intelligence analyst get a lenient sentence in the biggest leak of secret data in U.S. history.

Manning, 25, was convicted last week of charges that included espionage and theft for releasing more than 700,000 documents, battlefield videos and diplomatic cables to WikiLeaks, a pro-transparency website.

Rear Admiral Kevin Donegan, operations director from 2010 to 2012 of the U.S. Central Command, which includes Iraq and Afghanistan, testified that a task force was set up to assess the risk and damage for U.S. military forces and their allies.

"This was not a small operation," he said.

The U.S. military tried to warn anyone who could be identified by enemy forces through WikiLeaks that "they were potentially in jeopardy," Donegan said.

WikiLeaks also endangered Afghan villages that might be perceived as friendly toward U.S. or coalition forces, Donegan said.

He added that "there was absolutely an impact" on the U.S. government from the released diplomatic cables.

Under questioning from defense attorney Major Thomas Hurley, Donegan said he was unable to name anyone who became a casualty of reprisal because of the WikiLeaks publications.

The court went into a closed session to hear classified information from Donegan. Major General Kenneth McKenzie, a senior staff member at the Marine Corps headquarters, also testified behind closed doors.

Manning, of Crescent, Oklahoma, was convicted of releasing the files while working as a low-level intelligence analyst in Iraq in 2010. He was found not guilty of the most serious count of aiding the enemy, which carried a sentence of life without parole.

In setting Manning's sentence, Colonel Denise Lind, the military judge who convicted him, is determining how much damage he did.

Manning's lawyers have portrayed him as naive but well-intentioned. They argue the soldier's aim was to provoke a broader debate on U.S. military policy, not to harm anyone.

(Editing by Ian Simpson and Gunna Dickson)

[Image via Agence France-Presse]