U.S. Homeland Security chief Jeh Johnson has ordered tighter security at federal government buildings in Washington and other major cities.
Johnson said Tuesday that the reason for the enhanced presence of federal protective officers was "self-evident."
Terrorist groups have called for attacks inside the United States and elsewhere, including strikes on law enforcement and other government officials.
Johnson said world events, including two attacks in Canada last week, dictated heightened vigilance in protecting U.S. government installations.
Johnson did not mention specific threats and said the increased security would shift from location to location.
U.S. officials have said they are concerned about possible attacks from so-called "lone wolf" terrorists — individuals inspired by extremist ideology but without direct links to well-known terrorist groups.