(Reuters) The U.S. National Security Agency’s bulk collection of phone records provides only minimal benefits to countering terrorism, is illegal and should end, a federal privacy watchdog said in a report to be released on Thursday and reviewed by Reuters.

The watchdog agency’s report could further complicate efforts by both President Barack Obama and Congress to come up with possible reforms to NSA eavesdropping programs, which have been under harsh scrutiny in the wake of revelations by former NSA contractor Edward Snowden.

The Privacy and Civil Liberties Oversight Board, which produced the new report, is an independent government agency within the executive branch that advises the President and Congress on how to ensure that counterterrorism operations also protect Americans’ privacy.

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