WASHINGTON (Reuters) - The whistle-blowing website WikiLeaks may have blood on its hands, the Pentagon said on Thursday, warning its unprecedented leak of secret U.S. military files could cost lives and damage trust of allies.

An Army intelligence officer, already under arrest, is at the center of an investigation into the leak of more than 90,000 secret records to WikiLeaks, one of the biggest security breaches in U.S. military history, U.S. officials have said.

Defense Secretary Robert Gates declined to comment on the probe but said he could not rule out more leaks of classified information. He also announced plans to tighten access to sensitive intelligence data.

“I don’t know whether there is anyone else out there that is a party to this,” Gates said at the Pentagon in his first public comments since Sunday’s publication of the documents.

Admiral Mike Mullen, who as chairman of the Joint Chiefs of Staff is the top U.S. military officer, lashed out at WikiLeaks’ founder Julian Assange, who says he aims to expose corporate and government corruption.

“Mr. Assange can say whatever he likes about the greater good he thinks he and his source are doing,” Mullen said. “But the truth is they might already have on their hands the blood of some young soldier or that of an Afghan family.”

Gates said he did not know whether Assange should face criminal prosecution or whether WikiLeaks should be treated like a media organization protected by free speech rights under the U.S. Constitution. “I think that’s a question for people who are more expert in the law than I am,” he said.

But asked about a possible broadening of the criminal investigation to include WikiLeaks, Gates said he had asked the FBI to assist the Army’s probe to ensure that the investigation “can go wherever it needs to go.”

President Barack Obama and military top brass have played down any revelations from the leaked documents, which have fanned doubts in Washington about the unpopular and costly nine-year-old war.

Wikileaks founder Julian Assange holds up a copy of a newspaper during a press conference at the Frontline Club in central London, July 26, 2010. REUTERS/Andrew Winning

June was the deadliest month for foreign troops since the start of the conflict in 2001 and U.S. officials warn they expect casualties will keep rising over the summer.

U.S. CONTACTS AT RISK

Obama met his national security team at the White House on Thursday and officials said the WikiLeaks case was discussed.

Gates, a former CIA director, told reporters his biggest concern was that Afghans and other allies would no longer trust the United States to keep their secrets safe. The documents include intelligence reports and expose names of contacts.

“I spent most of my life in the intelligence business, where the sacrosanct principle is protecting your sources,” Gates said.

“It seems to me that, as a result of this massive breach of security, we have considerable repair work to do in terms of reassuring people and rebuilding trust, because they clearly -- people are going to feel at risk.”

He said there were technological solutions to tighten security of classified military networks. One defense official suggested possible measures could include deactivating computer functions used to download data onto portable devices, like CDs or thumb-drives.

Beyond exposing U.S. contacts, the leaked documents also threw an uncomfortable spotlight on links between Pakistan’s Inter-Services Intelligence agency and insurgents who oppose U.S. troops in neighboring Afghanistan.

Mullen acknowledged some ties remained but said Islamabad was “strategically shifting” against insurgents.

“There have been elements of the ISI that have ... a relationship with extremist organizations and that we, you know, we consider that unacceptable. In the long run I think that the ISI has to strategically shift,” he said.

“And they are strategically shifting. That doesn’t mean that they are through that shift at all.”

The Army investigation into the incident has focused on Army specialist Bradley Manning, who was already charged earlier this month with leaking information previously published by WikiLeaks, U.S. defense officials say.

Manning is awaiting trial on charges of leaking a classified video showing a 2007 helicopter attack that killed a dozen people in Iraq, including two Reuters journalists.

Neither Manning nor anyone else has been named as a suspect in the latest leak and investigators are not ruling out the involvement of multiple individuals.