US president George Bush signed a new law on Sunday that gives the US National Security Agency the right to tap electronic communications that start, end or pass through the US, without a warrant.
The move legalised what the NSA was already doing in secret.
The legislation contains few of the checks and balances that civil liberties advocates had wanted to ensure that warrantless surveillance did not result in unchecked snooping on innocent Americans in the US, said the Center for Democracy and Technology, which strongly opposed the measure.
The original legislation aimed to give the US the right to monitor exchanges between suspected terrorists based in the US and their overseas correspondents. But this appears to give the NSA carte blanche to monitor all traffic in and out of the US, provided the surveillance is based in the US, according to sources who have examined the legislation, the New York Times reported.
The new law allows the government to force communications carriers to give them customer lists and logs, and disallows law suits from affected customers.