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The Pentagon has demanded that Wikileaks remove a trove of secret documents on the Afghanistan war from its website and cancel plans to publish anything more it holds.

But a Pentagon spokesman acknowledged the already-leaked documents' viral spread across the internet made it unlikely they could ever be quashed.

"We are asking them to do the right thing," spokesman Geoff Morrell said.

Wikileaks called Mr Morrell "obnoxious" and appealed for public donations.

The US military documents released last month detail civilian deaths, friendly-fire episodes and other ground-level incidents.

They include allegations the Pakistani intelligence service has backed the Taliban insurgents' fight against the US-led coalition and the Afghan government, and indicate Taliban fighters have acquired surface-to-air missiles.

The US maintains the documents' leak puts coalition soldiers and Afghans, especially informers, at risk.

Wikileaks has an additional 15,000 secret documents it has not released, and the Pentagon has no authority to stop the website publishing those too.

Mr Morrell said the Pentagon had not been in contact with Wikileaks or its founder, Julian Assange.

"We're not looking to have a conversation about harm minimization," Mr 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."

On the Wikileaks Twitter feed, Mr Morrell's demand was described as coming close to a "formal threat".

The Pentagon is investigating whether Army Pfc Bradley Manning leaked the documents.

Pfc Manning is already jailed in the US awaiting trial on charges he gave the website video of a deadly helicopter attack in Iraq that killed a Reuters photographer.