SAN DIEGO, California (Reuters) - The Pentagon on Thursday told WikiLeaks it would be the “height of irresponsibility” if it went through with a new threat to publish outstanding documents it had on the Afghan war.

Amid news reports that WikiLeaks plans to soon release about 15,000 documents it had held back last month, Pentagon spokesman Geoff Morrell repeated a U.S. demand for the whistle-blower site to expunge all classified material from the Internet and return the material it had to the U.S. government.

“It is hard to believe anything WikiLeaks says, but our position on this matter should be well-known by now to everyone,” said Morrell, who was traveling with U.S. Defense Secretary Robert Gates on a trip to California.

WikiLeaks 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.

“If they were to publish any additional documents after hearing our concerns about the harm it will cause our forces, our allies and innocent Afghan civilians it would be the height of irresponsibility. It would compound a mistake that has already put far too many lives at risk,” said Morrell.

Earlier, Gates was asked about the impact of the leak by a sailor aboard the USS Higgins which is docked in San Diego.

“There are very serious operational consequences. There are the names of a lot of Afghans who have worked with us and helped us in those documents,” Gates said.

He said the documents released last month conveyed a huge amount of information and U.S. tactics, techniques and procedures, including where the United States was vulnerable.

“We know from intelligence that both the Taliban and al Qaeda have given directions to comb those documents for information and so I think the consequences are potentially very severe,” Gates said.

“We don’t have specific information of an Afghan being killed yet because of them (the documents) -- but put the emphasis on the word ‘yet’,” he told the sailors.