The US has condemned as "irresponsible" the leak of 90,000 classified military records, saying their publication could threaten national security.
The documents released by the Wikileaks website include details of killings of Afghan civilians unreported until now.
The records also show Nato concerns that Pakistan's ISI intelligence agency is helping the Taliban in Afghanistan, an accusation Islamabad has denied.
The Pentagon said it might take weeks to ascertain what damage had been done.
Calling their release a "criminal act", spokesman Col Dave Lapan said officials were reviewing the documents to determine "whether they reveal sources and methods" and might endanger US and coalition personnel.
A spokesman for Afghan President Hamid Karzai said he was "shocked" at the scale of the leaks, but thought that "most of this is not new".
Mr Karzai's office later said the documents "clearly support and verify Afghanistan's all-time position that success over terrorism does not come with fighting in Afghan villages, but by targeting its sanctuaries and financial and ideological sources across the borders".
The huge cache of classified papers - posted by Wikileaks as the Afghan War Diary - is one of the biggest leaks in US history. It was also given in advance to the New York Times, the Guardian and the German news magazine, Der Spiegel.
The founder of Wikileaks, Julian Assange, said he had no reason to doubt the reliability of the reports.
"When we publish material, what we say is: the document as we describe it is true," he said at a news conference in London.
"We publish CIA reports all the time. They are legitimate reports, but they don't mean the CIA is telling the truth."
Mr Assange said there was no one overarching revelation to come out of the cache.
"The real story of this material is that it's war - it's one damn thing after another," he said.
"It is the continuous small events, the continuous deaths of children, insurgents, allied forces, the maimed people. Search for the word 'amputation' in this material, or 'amputee', and there are dozens and dozens of references."
He compared the impact of the released material to the opening of the archives of the East German secret police, the Stasi.
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 January 2004 to December 2009, before President Barack Obama "announced a new strategy with a substantial increase in resources for Afghanistan".
But Mr Assange was sceptical, saying: "A new policy by Obama doesn't mean new practice by the US military."
He also said Wikileaks had "tried hard to make sure that this material does not put innocents at harm".
"All the material is over seven months old so is of no current operational consequence, even though it may be of very significant investigative consequence."
After being asked repeatedly by reporters whether he believed some of the incidents described in the documents constituted war crimes, Mr Assange said: "It is up to a court to decide, clearly, whether something is, in the end, a crime."
"That said, prima facie, there does appear to be evidence of war crimes in this material," he added.
He cited as an example an attack in June 2007 by a secret US special forces unit, Task Force 373, which used a Himars (High Mobility Artillery Rocket System) to begin a raid on a compound where a senior al-Qaeda leader, Abu-Laith al-Libi, was thought to be hiding. Seven children died.
The Nato-led International Security Assistance Force (Isaf) acknowledged the deaths of the children at the time, but stated that coalition troops had attacked because of "nefarious activity" there.
It did not mention they had targeted al-Libi nor used a Himars before any shots had been fired at them, and has not commented on the details included in the Wikileaks papers.
Pakistan's government, meanwhile, denied claims its Inter-Services Intelligence (ISI) agency backed the Taliban in the war in Afghanistan.
One of the leaked documents refers to an alleged meeting in December 2006 between insurgents and the former ISI chief, Lt Gen Hamid Gul, during which he claimed to have dispatched three men to Kabul to carry out attacks.
He dismissed the Wikileaks material as "pure fiction which is being sold as intelligence".
"It's not intelligence," Gen Gul, who ran the agency from 1987 to 1989, told the BBC. "It may have a financial angle to it but more than that it is not hardcore [intelligence]. I'm an old veteran. I know."
"It is all wrong. It's precisely as their intelligence regarding Saddam Hussein keeping weapons of mass destruction in his closet," he added. "This is all based on falsehood. That is why they are not winning, because they have no cause."
The Pakistani presidential spokeswoman, Farahnaz Ispahani, said the leaks might be an attempt to sabotage the new strategic dialogue between the US and Pakistan.
The reports also suggest:
- The Taliban has had access to portable heat-seeking missiles to shoot at aircraft
- A secret US special forces unit, Task Force 273, has been engaged on missions to "capture or kill" top insurgents listed on a Joint Priority Effects List (JPEL)
- Many civilian casualties - caused by Taliban roadside bombs and Nato missions that went wrong - have gone unreported
- Iran is engaged in an extensive covert campaign to arm, finance and equip the Taliban and Afghan warlords allied to al-Qaeda
The head of the Foreign Relations Committee in the US Senate said the leak came at a "critical stage" for US policy in the region.
"However illegally these documents came to light, they raise serious questions about the reality of America's policy toward Pakistan and Afghanistan," Democratic Senator John Kerry said.
UK Foreign Secretary William Hague said he did not think the leaks would damage the international effort in Afghanistan.
Wikileaks says it delayed the release of about 15,000 reports from the archive as part of a "harm minimisation process demanded by our source".
Earlier this year, Wikileaks posted a video on its website which it said showed the killings of civilians by a US military helicopter in Baghdad in 2007.
A US army intelligence analyst, Specialist Bradley Manning, is awaiting trial on charges including releasing classified information.
A former hacker, Adrian Lamo, said Spc Manning boasted to him about handing over military videos and 260,000 classified US embassy messages to Wikileaks.
Wikileaks has refused to identify its source for the video or the US military documents.
Meanwhile, Nato said an investigation had found "no evidence" that as many as 52 civilians died in an air strike in Helmand province on Friday.
President Karzai's office had said Afghan intelligence believed coalition forces had killed women and children in the village of Rigi. The BBC also spoke to villagers who said they had witnesses the incident.
"Any speculation at this point of an alleged civilian casualty in Rigi Village is completely unfounded," Isaf Communication Director Rear Adm Greg Smith said in a statement.