BBC. The United States has condemned as "irresponsible" the leak of 90,000 military records, saying publication could threaten national security.
The documents released by the Wikileaks website include details of killings of Afghan civilians unreported until now.
Three news organisations had advance access to the records, which also show Nato concerns that Pakistan and Iran are helping the Taliban in Afghanistan.
Pakistan has denied claims its intelligence agency backed the Taliban.
In a statement, US National Security Adviser Gen James Jones said such classified information "could put the lives of Americans and our partners at risk".
He said the documents covered the period from 2004 to 2009, before President Obama "announced a new strategy with a substantial increase in resources for Afghanistan".
The huge cache of classified papers - described as one of the biggest leaks in US military history - was given to the New York Times, the Guardian and the German news magazine, Der Spiegel.
Pakistan denied claims its intelligence agency, the ISI, backed the Taliban in the war in Afghanistan.
Pakistan's ambassador in Washington said the "unprocessed" reports did "not reflect the current onground realities".
"I think that the American leadership knows what Pakistan is doing," Husain Haqqani told the BBC.
"We have paid a price in treasure and in blood over the last two years. More Pakistanis have been killed by terrorists, including our military officers and intelligence service officials.
"We are not going to be distracted by something like this," he said.
The reports also suggest:
• The Taliban has had access to portable heat-seeking missiles to shoot at aircraft.
• A secret US unit of army and navy special forces has been engaged on missions to "capture or kill" top insurgents.
• Many civilian casualties - caused by Taliban roadside bombs and Nato missions that went wrong - have gone unreported.
The BBC's David Loyn, in Kabul, says these revelations will be "hugely embarrassing" for the White House, who are "infuriated" by the leak.
"These documents show aid that has not worked; political policy that was naïve at the beginning; and it shows quite how difficult a foe the Taliban have been. The Americans have misunderstood that until today," he says.
'Civilian deaths'
Another US official said that Wikileaks - which specialises in making public untraceable material from whistleblowers - was not an objective news outlet and described it as an organisation that opposes US policy in Afghanistan.
But the head of the Foreign Relations Committee in the US Senate said that "however illegally these documents came to light, they raise serious questions about the reality of America's policy toward Pakistan and Afghanistan".
"Those policies are at a critical stage and these documents may very well underscore the stakes and make the calibrations needed to get the policy right more urgent," said Democrat Senator John Kerry.
Wikileaks is releasing the set of documents under the title Afghan War Diary.
It says it has delayed the release of about 15,000 reports from the archive as part of a "harm minimisation process demanded by our source".
The Guardian and the New York Times say they had no contact with the original source of the leak, but spent weeks crosschecking the information.
David Leigh from the Guardian said the documents made a strong impression on him.
"The picture that came out was of hundreds of attacks on civilians...not only the victims of airstrikes...but also, time and again, perfectly innocent drivers and motorcyclists, sometimes women and children, got shot by convoys or patrols just for getting too close."
The reports come as Nato says it is investigating reports that as many as 45 civilians died in an air strike in Helmand province on Friday.
Although an initial Nato investigation found no evidence, a BBC journalist visiting Regey village spoke to several people who said they had witnessed the incident.
They said the attack had come in daylight as dozens sheltered from fighting in nearby Joshani.
A Nato spokesman said international forces went to great lengths to avoid civilian casualties.
"The safety of the Afghan people is very important to the International Security Assistance Forces," Lt Col Chris Hughes added.