Medea Benjamin, founder of the protest group Code Pink, at a demonstration against President Barack Obama and the NSA on Jan. 17. Larry Downing/Reuters

The National Security Agency's bulk collection of phone records provides only minimal counterterrorism benefits, is illegal and should end, a federal privacy watchdog said in a report to be released Thursday.

The report could further complicate efforts by 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 report, seen by some media outlets, 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.

In its report, the board called on the government to end the NSA program that collects U.S. telephone records in bulk and to purge the data it had collected, adding ammunition to efforts by privacy advocates to win new restrictions on government surveillance programs.

"The Section 215 bulk telephone records program (referring to the section of the Patriot Act that defines it) lacks a viable legal foundation under Section 215, implicates constitutional concerns under the First and Fourth Amendments, raises serious threats to privacy and civil liberties as a policy matter, and has shown only limited value," the board said in its report.

In its report, the review panel, which included former White House and intelligence officials, also raised questions about the value of telephone metadata collection in producing counterterrorism breakthroughs. The panel recommended that metadata collection continue, but that data should henceforth be stored by telephone companies or an independent third party.

In a speech last Friday, Obama agreed that the storage of telephone metadata should be moved out of government hands, but did not call for an outright halt to the collection of such data by the NSA. The president also said that such data should not be searched without judicial approval.