The Pentagon has launched an investigation to find out who leaked tens of thousands of classified documents on the war in Afghanistan.

Some 91,000 classified documents were leaked in what is one of the largest security breaches in US military history.

US defence officials said the person behind the leak appeared to have 'secret' clearance and access to sensitive documents on the Afghan war.

More leaks were possible, officials acknowledged.

The Pentagon said its review of the documents being made public by the WikiLeaks website would take 'days if not weeks' and that it was too soon to assess any damage to national security.

However, US military officials have played down any revelations within the documents revealed so far.

They said they appeared to be low-level assessments that largely confirm the military's publicly stated concerns about the Afghan war.

Allegations

The documents dating from 2004 to 2009 were released to The New York Times, The Guardian and Germany's Der Spiegel by the website WikiLeaks, which posted them on Sunday.

The most controversial allegations centre on claims that Pakistan, a key US ally in the turbulent region, allows its spies to meet directly with the Taliban.

According to the Times, Pakistan agents and Taliban representatives meet regularly 'in secret strategy sessions to organise networks of militant groups that fight against American soldiers in Afghanistan, and even hatch plots to assassinate Afghan leaders.'

The files also maintained that the deaths of innocent civilians have been covered up, and that Iran is funding Taliban militants, eight years after the 2001 US-led invasion ousted the radical Islamic regime from power.

Reaction

The revelations triggered outrage, with a top NATO general calling for increased vigilance against such leaks as the White House criticised them as 'irresponsible'.

White House spokesman Robert Gibbs warned that the names of service personnel and military operations were now in the public domain.

'In terms of broad revelations, there aren't any that we see in these documents,' Mr Gibbs said, pointing out that most of the period covered by the leaks was during the previous Bush administration.

Britain, which has some 9,500 troops in Afghanistan, said yesterday it regretted the leak, as Pakistan said the reports were 'skewed' and not based on the reality on the ground.

WikiLeaks founder Julian Assange defended the decision to publish the leaked files.

He said they showed 'thousands' of war crimes may have been committed in Afghanistan.

'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 said, citing a missile strike on a house which killed seven children.