An aerial view of the Pentagon (lower left), Potomac River (C) and Washington Monument in Washington August 31, 2010. (Photo: REUTERS)

Reuters has reviewed more than 50 of these documents, labeled "Secret" and "Top Secret", that first appeared on social media websites in early March and purportedly reveal details of Ukrainian military vulnerabilities and information about allies including Israel, South Korea and Turkey.

US officials have said some giving battlefield casualty estimates from Ukraine appeared to have been altered to understate Russian losses.

Two US defense officials, speaking on condition of anonymity because of the sensitivity of the matter, said the Pentagon was examining procedures governing how widely some of the most sensitive U.S. secrets are shared.

The Pentagon on Sunday said in a statement that an interagency effort was assessing the impact the photographed documents could have on US national security as well as that of close American allies, a standard procedure known as "damage assessment" for leaks of classified information.

Some national security experts and US officials say they currently suspect that the leaker could be American, given the breadth of topics covered by the documents, but they do not rule out pro-Russian actors.