US to "turn over every rock" to find source of leak - Pentagon chief

US Defense Secretary Lloyd Austin has said the United States will investigate the recent purported leak of classified documents until the source is found.

Reuters has reviewed more than 50 of the documents, labelled "Secret" and "Top Secret," that first appeared on social media sites in March and supposedly reveal details of the military capabilities of some US allies and adversaries.

Reuters has not independently verified the documents' authenticity.

"We will continue to investigate and turn over every rock until we find the source of this and the extent of it," Austin said during a press conference at the State Department.

Austin, the first senior US official to comment on the leak, said the Pentagon was aware that documents had been posted dated February 28 and March 1, but was not sure if there were other documents that had been online before.

"These are things that we will find out as we continue to investigate," Austin added.

US Central Intelligence Agency Director William Burns, speaking later at Rice University in Texas, called the leaks "deeply unfortunate," but did not give details on what he said were "quite intense" investigations by the Pentagon and the Justice Department.

"We need to learn lessons from that, as well, about how we can tighten procedures," Burns said.

Investigators are working to determine what person or group might have had the ability and motivation to release the intelligence reports. The leaks could be the most damaging release of US government information since the 2013 publication of thousands of documents on WikiLeaks.

Milancy Harris, the deputy undersecretary of defence for intelligence and security, is leading the Pentagon's review to assess the potential impact of the leaked documents, a US official told Reuters.

Some of the most sensitive information is purportedly related to Ukraine's military capabilities and shortcomings, and one document mentions the small number of Western special forces troops in the country.

The Justice Department has opened a criminal investigation into the disclosure of the documents.

Some national security experts and US officials say they suspect the leaker could be American, given the breadth of topics covered by the documents. More theories could develop as the investigation progresses, they said.