The Pentagon demanded Thursday that a website that solicits leaked government secrets cancel any plan to publish more classified military documents and pull back tens of thousands of secret Afghan war logs already posted on the Internet.

The demand, which the Pentagon has no independent power to enforce, is primarily aimed at preventing release of approximately 15,000 secret documents that the website WikiLeaks has said it is holding. The Pentagon also hopes to stop WikiLeaks from making public the contents of a mammoth encrypted file recently added to the site. Contents of that file remain a mystery.

"We are asking them to do the right thing," Pentagon press secretary Geoff Morrell said. "I don't know that we're very confident they'll have a change of heart."

WikiLeaks did not immediately reply to calls and e-mails seeking comment on the Pentagon's demand, although on its Twitter feed the group seemed dismissive, calling Morrell "obnoxious" and saying his demand was tantamount to a "formal threat."

WikiLeaks posted more than 76,900 classified military and other documents, mostly raw intelligence reports from Afghanistan, on its website July 25. The 15,000 additional documents are apparently related to that material.

The documents leaked so far illustrate the frustration of U.S. forces in fighting the protracted Afghan conflict and revived debate over the war's uncertain progress. The White House angrily denounced the leaks, saying they put the lives of Afghan informants and U.S. troops at risk.

"The Defense Department demands that WikiLeaks return immediately to the U.S. government all versions of documents obtained directly or indirectly from the Department of Defense databases or records," Morrell said.

He called the material stolen property, but would not address whether the demand is a prelude to legal action against the website or others. Morrell spoke at a Pentagon press conference that amounted to a televised public appeal to the secretive site and its editor in chief, Julian Assange.

Generally speaking, WikiLeaks has so far struck an uncompromising tone, with Assange telling journalists in London last week that he had no obligation to the U.S. military and found the very notion of "national security" ridiculous.

An Army private, Bradley Manning, is jailed on suspicion of leaking classified material to WikiLeaks in a previous case. He is a "person of interest" in the latest release, Morrell said.

As a practical matter, the Pentagon has little if any hope that it can recapture all electronic forms of the documents already placed online and since downloaded and examined by countless people.

"The genie is out of the bottle," Morrell acknowledged later, but he said WikiLeaks would make matters worse by releasing more information.

The Pentagon has had no direct contact with WikiLeaks about possible efforts to redact those documents to make them less of a security threat, Morrell said, and he ruled out such an exercise.

"We're not looking to have a conversation about harm minimization," Morrell said. "We're looking to have a conversation about how to get these perilous documents off the website as soon as possible, return them to their rightful owners and expunge them from their records."

The Pentagon has some idea what the 15,000 unpublished documents contain, he said. U.S. intelligence and security officials appear worried that the unpublished material contains more damaging secrets than were contained in the low-level military intelligence reports first released.

WikiLeaks claimed Wednesday that the group had always sought — and was still seeking — to open a line of communication with the Defense Department.

"WikiLeaks have wanted that for some time," WikiLeaks told The Associated Press. But it added that the Pentagon had so far made no attempt to contact the whistleblower website directly.

Also hanging fire are secret State Department documents that Manning is suspected of obtaining.

State Department spokesman P.J. Crowley told reporters Thursday that the government thinks WikiLeaks has classified State Department material that it has not released.

"Certainly as a government, we would like to see all documents returned, whether they're military cables, whether they're State Department cables. This is classified information that WikiLeaks does not have a right to possess," Crowley said.

WikiLeaks has posted a huge encrypted file named "Insurance" to its website, raising the possibility that the organization may be prepared to release another wave of secret material if the government attempts to block the site or target its operators.

Bloggers have noted that the file is 20 times the size of the batch already released.

WikiLeaks wouldn't comment Thursday on the 1.4 gigabyte file beyond a vague reference to "security procedures."

Assange said little more in his response to the same line of questioning in a television interview with independent U.S. news network Democracy Now!

"I think it's better that we don't comment on that," Assange said, according to the network's transcript of the interview. "But, you know, one could imagine in a similar situation that it might be worth ensuring that important parts of history do not disappear."

Assange has expressed concern over his safety in the past, complaining of surveillance and telling interviewers that he's been warned away from visiting the United States.

The Pentagon has a team of about 80 intelligence experts combing the documents already released for information that Taliban insurgents or others could use to hone their tactics against U.S. force or target informants. That team, which includes military intelligence analysts and others culled from the nation's vast constellation of intelligence agencies, could soon grow to as many as 125 people, Morrell said.

— Associated Press writers Pauline Jelinek and Matthew Lee in Washington and Raphael G. Satter in London contributed to this report.