Federal offices have until 19 June to review Website content for "sensitive but not classified" materials and report back to the US Office of Homeland Security, according to a White House memo.
White House spokesman Jimmy Orr said the material could include anything that would threaten national defence or aid in the construction of "weapons of mass destruction".
Orr would not give any specific examples, nor would he say whether the public could access the information elsewhere, such as through the Freedom of Information act.
Ari Schwartz, policy analyst for the Washington-based Center for Democracy and Technology, believed the White House had created a "huge blanket" that could be cast over any type of information that the US government wanted to hide. Instead of making installations such as dams and nuclear power plants more secure, the administration is hiding information about these sites.
"They are blaming the messenger for the problems that are out there," he said.
Before 11 September, President George Bush admitted that his administration would not be as open as past presidencies, and he has blocked information from the Reagan administration that would have been made public this year.
Orr stressed that the review follows on from a policy created by Bush's predecessor, Bill Clinton.