By late Thursday night, U.S. Director of National Intelligence James Clapper moved quickly to defend the PRISM program, saying the related reports published by the two newspapers contained " numerous inaccuracies."

"Information collected under this program is among the most important and valuable foreign intelligence information we collect, and is used to protect our nation from a wide variety of threats," said Clapper in a statement.

Clapper insisted that the program is subject to oversight by the Foreign Intelligence Surveillance Court, the Executive Branch and Congress, involving "extensive procedures" to ensure that " only non-U.S. persons outside the U.S. are targeted."

He also warned that "the unauthorized disclosure of information about this important and entirely legal program is reprehensible and risks important protections for the security of Americans."

U.S. Lawmakers will review the domestic surveillance laws after the reports of the NSA's classified program have provoked an uproar on Capitol Hill.