Four US soldiers plotted to assassinate Barack Obama and overthrow the government, a court has heard.

Prosecutors in Georgia said they formed an anarchist militia within the military.

One, private Michael Burnett, has pleaded guilty to manslaughter and gang charges in the killings last December of former soldier Michael Roark and his girlfriend, 17-year-old Tiffany York.

Burnett said that Roark, who had just left the army, knew of the militia group's plans and was killed because he was "a loose end."