NEW YORK -- The unprecedented and unwarranted bulk collection of the entire U.S. population's phone records by the government is illegal because it wasn't authorized by Congress, a federal appeals court said Thursday as it asked legislators to balance national security and privacy interests.

A three-judge panel of the 2nd U.S. Circuit Court of Appeals in Manhattan permitted the National Security Agency program to continue temporarily as it exists, but it all but pleaded for Congress to better define where boundaries exist or risk "invasions of privacy unimaginable in the past."

The appeals court said if the government is correct in its legal arguments, it also could justify storing in bulk metadata associated with Americans' financial records, medical records and electronic communications, including email and social media information.

An opinion written by Circuit Judge Gerald Lynch said Congress could "profoundly alter the legal landscape" after debate.

"Congress cannot reasonably be said to have ratified a program of which many members of Congress -- and all members of the public -- were not aware," it said.

"The statutes to which the government points have never been interpreted to authorize anything approaching the breadth of the sweeping surveillance at issue here," the court said. "The sheer volume of information sought is staggering."

A lower court judge in December tossed out an American Civil Liberties Union lawsuit, saying the program was a necessary extension to security measures taken after the Sept. 11, 2001, terrorist attacks.

U.S. Attorney General Loretta Lynch said the government is reviewing the court's decision. She added the June 1 expiration of the Patriot Act provisions provides opportunities to reauthorize the program "in a way that does preserve its efficacy and protect privacy."