The FBI has been asked to look into how Wikileaks got hold of more than 90,000 classified US military documents.

Defence secretary, Robert Gates, speaking from the Pentagon, has said the question of how the leaks occurred needs to be investigated by the FBI.

Gates referred to the leaks as "potentially severe and dangerous," saying: "We need to ensure the investigation goes wherever it needs to go. Having the FBI involved as a partner is very important."

Wikileaks, the organisation that accessed and published the documents on its website, has been criticised by many officials in the US military, including Admiral Mike Mullen, the chairman of the joint chiefs of staff.

On Thursday Mullen said he had been angered by the carelessness of Wikileaks' founder, Julian Assange, saying the leaking of the documents had put NATO soldiers and Afghan citizens working with NATO at risk.

He said Assange might have already "spilt the blood of some young soldier or an Afghan family."

Afghanistan's president, Hamid Karzai, also expressed concern for the safety of Afghans named in the documents.

He said the leaks had been "shocking" and "irresponsible," putting lives in danger.

Assange has already said Wikileaks has other sensitive material which will eventually be placed on the website.