The White House responded with a strong condemnation of the leak, saying it could threaten national security and endanger the lives of Americans and those of its allies.
''These irresponsible leaks will not impact our ongoing commitment to deepen our partnerships with Afghanistan and Pakistan,'' President Barack Obama's National Security Advisor Jim Jones said in a statement.
Obama is sending thousands of additional troops into Afghanistan to combat a resurgent Taliban, despite disquiet at home over rising military casualties and debate within his own administration about the right way to fight the 9-year war.
The New York Times reported the 91,000 documents, collected from across the U S military in Afghanistan, showed Pakistan actively collaborating with the Afghan insurgency.
"The documents ... suggest that Pakistan, an ostensible ally of the United States, allows representatives of its spy service to meet directly with the Taliban in secret strategy sessions to organize networks of militant groups that fight against American soldiers in Afghanistan, and even hatch plots to assassinate Afghan leaders," the Times said.
The documents were also made available to Britain's The Guardian newspaper and German weekly Der Spiegel.
A summary of the documents is available here, 2004-2010, along with a link to the webpage where WikiLeaks said the documents would be posted later on Sunday.
Jones said the documents covered a period from January 2004 to December 2009, when Obama launched his new Afghan strategy.
"President Obama announced a new strategy with a substantial increase in resources for Afghanistan, and increased focus on al Qaeda and Taliban safe-havens in Pakistan, precisely because of the grave situation that had developed over several years," Jones said.
"We know that serious challenges lie ahead, but if Afghanistan is permitted to slide backwards, we will again face a threat from violent extremist groups like al Qaeda who will have more space to plot and train," he said.
WikiLeaks promotes the leaking of information to fight government and corporate corruption. Earlier this year it leaked a classified video showing a 2007 helicopter attack that killed a dozen people in Iraq, including two Reuters journalists.
Pakistan's ambassador to the United States Husain Haqqani said leaking unprocessed reports from the battlefield was irresponsible.
"These reports reflect nothing more than single source comments and rumors, which abound on both sides of the Pakistan-Afghanistan border and are often proved wrong," he said in a statement.
There have been other claims of Pakistani meddling in Afghanistan. Earlier this month the London School of Economics published a report that strongly suggested support for the Taliban was the "official policy" of Pakistan's Inter-Services Intelligence (ISI) agency. Pakistan strongly denied the claim.
