US Secretary of Defense Lloyd Austin says Washington intends "to investigate and turn over every rock" regarding the unauthorized disclosure of sensitive and classified government material.

Images of what are said to be US government documents concerning Russia's invasion of Ukraine have been found circulating on social media.

The Defense Department is investigating the possibility that the documents include highly classified material.

Austin told reporters on Tuesday that he was first briefed on the leak on April 6. He said, "We take this very seriously" and he insisted he has been conferring with senior department leaders daily on the government's response, and ordered an urgent cross-department effort.

He said they have referred the matter to the Justice Department, which has opened a criminal investigation.

Secretary of State Antony Blinken said the US government has engaged with allies and partners at high levels over the past days, aiming to reassure them about its commitment to safeguarding intelligence and security partnerships.

US media outlets say some of the material circulating online suggests that Washington is conducting surveillance activities on its friends and allies.