Afghan former militants attend a ceremony as they surrender under a US-backed Afghan government anmesty progam, in Herat western Afghanistan, 26 July 2010. EPA/JALIL REZAYEE

White House spokesman Robert Gibbs confirmed that an investigation began last week, ahead of the publishing of the documents by WikiLeaks, a whistle-blower website, Sunday night. Gibbs said the leak was a breach of federal law.

Whenever you have the potential for names and for operations and for programmes to be out there in the public domain, that it - besides being against the law - has the potential to be very harmful to those that are in our military, those that are cooperating with our military, and those that are working to keep us safe, Gibbs said.

While Gibbs said the documents provided no broad new revelations, he added that they pose problems because they reveal operational details, including names, programmes and logistics. He pointed out that the documents originated in the 2004-2009 period, before President Barack Obama revised the war strategy to intensify the fight against the Taliban and al-Qaeda.

The documents largely focus on concerns that Pakistans intelligence service maintained ties with the Taliban and other groups fighting US-led international forces in Afghanistan, and over civilian casualties. Gibbs said those two issues have already received a large amount of public attention through the media and public remarks by senior officials.

The release of the 91,000 documents is the largest since the Vietnam war-era Pentagon Papers were leaked to media in the 1970s, and the second time WikiLeaks has drawn the anger of the US government. In April, WikiLeaks released a classified video of a 2007 US helicopter attack in Baghdad that killed civilians. A US soldier has been arrested and is being held in Kuwait in connection with that case.

The US, British and Afghan governments all condemned the latest leak. State Department spokesman PJ Crowley said US diplomats contacted the governments of Afghanistan and Pakistan to provide advance warning of the release.

A key focus of the leak has been concerns expressed in the documents that Pakistans Inter-Services Intelligence agency, or ISI, continued working with the Taliban even as the United States formed an alliance with Pakistan along with billions of dollars in aid to fight terrorism. Gibbs said this has been a long-standing concern, while emphasizing that Pakistan has been an important partner in the war on terrorism, pointing to the military crackdown in Swat Valley and South Waziristan.

Obama delivered a strong message to the Pakistanis in March 2009 that they must do more to cooperate and would not receive a blank cheque.

Even as they make progress, we understand that the status quo is not acceptable and that we have to continue moving this relationship in the right direction, he said.

Details in the documents revealed some efforts to hide civilian casualties, as well as not publicly disclosing that the Taliban had acquired some sophisticated surface-to-air missiles.

WikiLeaks weeks ago gave the documents to the New York Times, the London-based Guardian and the German magazine Der Spiegel, after the three organizations agreed not to publish stories until Sunday night.