Pakistan was actively collaborating with the Taliban in Afghanistan while accepting US aid, new US military reports showed, a disclosure likely to increase the pressure on Washington's embattled ally.
The revelations by the organisation Wikileaks emerged as Admiral Mike Mullen, Chairman of the Joint Chiefs of Staff, warned of greater Nato casualties in Afghanistan as violence mounts over the summer.
It also came as the Taliban said they were holding captive one of two US servicemen who strayed into insurgent territory, and that the other had been killed. The reported capture will further erode domestic support for America's nine-year war.
Documents leaked by Wikileaks said representatives from Pakistan's Inter-Services Intelligence met directly with the Taliban in secret strategy sessions to organise militant networks fighting US soldiers.
The White House condemned the leak, saying it could threaten national security and endanger the lives of Americans. Pakistan said leaking unprocessed reports from the battlefield was irresponsible.
US national security adviser Jim Jones said the leak would not affect "our ongoing commitment to deepen our partnerships with Afghanistan and Pakistan".
The revelations were contained in more than 90,000 classified documents which US officials focused on the period leading to the launch of President Barack Obama's Afghan strategy last December, when he authorised deployment of 30,000 additional troops.
Violence in Afghanistan is at its highest of the 9-year-old war as the thousands of extra US troops step up their campaign to drive insurgents out of their traditional heartland in the south.
"As we continue our force levels and our operations over the summer ... we will likely see further tough casualties and levels of violence," Admiral Mullen told reporters in Kabul on Sunday.