The DHS says it wants to use the imagery to improve the quality of surveillance on borders, ports, bridges and other infrastructure.

'Police state'

Officials believe the satellites could identify and follow targets by their physical traits, and could be used to respond to disasters such as the 2001 attacks or Hurricane Katrina.

The Assistant Secretary of Homeland Security for intelligence analysis, Charles Allen, told the Washington Post the programme would be particularly useful in policing land and sea borders and in disaster planning.

Officials say use of the images for domestic law enforcement will be allowed only in limited cases, and a general ban on domestic spying by the CIA and the Pentagon will remain in force.

"This is not a system for tracking Americans," Mr Allen said.

Civil liberties activist Kate Martin, from the Center for National Security Studies, told the Wall Street Journal the program was akin to "Big Brother in the sky".

"They want to turn these enormous spy capabilities, built to be used against overseas enemies, on to Americans," Ms Martin said. "They are laying the bricks one at a time for a police state."

But Mr Allen dismissed privacy fears, saying: "Americans shouldn't have any concerns about it".