WASHINGTON (Reuters) - The Pentagon demanded on Thursday that whistle-blower web site WikiLeaks immediately hand over about 15,000 secret Afghan war records it had not yet published and erase material it had already put online.

Wikileaks founder Julian Assange speaks at a news conference at the Frontline Club in central London, July 26, 2010. REUTERS/Andrew Winning

“We are asking them to do the right thing,” said Pentagon spokesman Geoff Morrell in asking WikiLeaks to hand over the U.S. documents and delete material it had put on the Internet.

“We hope they will honor our demands,” he told reporters, adding that the only rightful owner of all the classified material in WikiLeaks’ possession was the U.S. government.

The whistle-blower site caused an uproar when it published more than 70,000 documents last month, at a time when U.S. public and congressional support for the nine-year war in Afghanistan is flagging.

The defense department said the leak -- one of the largest in U.S. military history -- put U.S. troops and Afghan informers at risk and WikiLeaks might already have blood on its hands.

WikiLeaks founder Julian Assange has said his group held back about 15,000 papers to protect innocent people from harm and Morrell said the appeal for documents to be returned was to prevent publication of material that could cause further damage.

Asked whether the Defense Department would take legal action against WikiLeaks if it refused to comply with the demand, Morrell declined to give specifics but said it was up to the FBI and Justice Department to decide how to proceed.

“If doing the right thing is not good enough for them (WikiLeaks), then we will figure out what other alternatives we have to compel them to do the right thing,” he added.

State Department spokesman P.J. Crowley also urged WikiLeaks to return leaked documents, including classified diplomatic cables they had obtained.

“Do we believe that WikiLeaks has additional cables? We do. Do we believe that those cables are classified? We do. And are they State Department cables? Yes,” he told reporters at the State Department briefing.

DAMAGE CONTROL

Morrell said public disclosure of the secret war documents, which included names of Afghan informers and covered the time from 2004 to December 2009, had already caused damage. The bid to get the information expunged from the web site and all the documents returned was aimed at minimizing further harm.

WikiLeaks also has a large, encrypted file on its web site titled “insurance” which has not been released to the public, and the Pentagon is jittery over the possible publication of more unknown material.

Morrell said a task force of about 80 government intelligence experts was painstakingly combing through the 70,000 or so documents already released and notifying foreign governments and others when material was seen as posing a risk.

The intelligence analysts had done about 400 initial “word searches” of the documents and were working around the clock to analyze the material in greater depth, Morrell added.

Morrell said the department had an idea of what might be in the 15,000 unreleased documents and was doing “prophylactic work” in case WikiLeaks ultimately posted this material. He declined to give details.

The U.S. investigation focuses on Bradley Manning, who worked as an Army intelligence analyst in Iraq. Manning is already under arrest and charged with leaking a classified video showing a 2007 helicopter attack that killed a dozen people in Iraq, including two Reuters journalists.

The New York Times, Der Spiegel and The Guardian newspaper were asked by WikiLeaks to review the documents before they were released on the Internet.