The Newsweek magazine's exclusive report, titled "Inside the Military's Secret Undercover Army", said that after a two-year investigation, it found "some 60,000 people now belong to this secret army, many working under masked identities and in low profile, all part of a broad program called 'signature reduction'". 

The clandestine force included special operations forces and military intelligence specialists, among others, who have carried out such "publicly unacknowledged operations" as wiretapping activities in certain regions and manipulating social media, Xinhua news agency quoted the report as saying. 

"What emerges is a window into not just a little-known sector of the American military, but also a completely unregulated practice," it said. 

The report added the program's total size is unknown, and its influence on military policies and culture is uninspected. 

"Congress has never held a hearing on the subject. And yet the military developing this gigantic clandestine force challenges US laws, the Geneva Conventions, the code of military conduct and basic accountability," it said.