By Dustin Volz

WASHINGTON (Reuters) - U.S. lawmakers are pressing the nation's top intelligence official to estimate the number of Americans ensnared in email surveillance and other such spying on foreign targets, saying the information was needed to gauge possible reforms to the controversial programs.

Eight Democrats and six Republicans made the request to Director of National Intelligence James Clapper in a letter seen by Reuters on Friday, reflecting the continued bipartisan concerns over the scope of U.S. data espionage.

"You have willingly shared information with us about the important and actionable intelligence obtained under these surveillance programs," wrote the lawmakers, all members of the U.S. House of Representatives' Judiciary Committee.

"Now we require your assistance in making a determination that the privacy protections in place are functioning as designed."

They requested that Clapper provide the information about data collected under a statute, known as Section 702, by May 6.

That law, set to expire at the end of 2017, enables an Internet surveillance program called Prism that was first disclosed in a series of leaks by former National Security Agency contractor Edward Snowden some three years ago.

Prism gathers messaging data from Alphabet's Google, Facebook, Microsoft, Apple and other major tech companies that is sent to and from a foreign target under surveillance.

Intelligence officials say data about Americans are "incidentally" collected during communication with a target reasonably believed to be living overseas. Critics see it as "back-door" surveillance on Americans without a warrant.

A recently declassified November opinion from the Foreign Intelligence Surveillance Court, a secretive body that oversees the legality of U.S. spy programs, rejected a constitutional challenge to rules permitting the FBI to access foreign intelligence data for use in domestic criminal investigations.

The Republican-controlled House has voted overwhelmingly since the Snowden leaks to require U.S. agencies obtain a warrant before searching collected foreign intelligence for data belonging to Americans, but those proposals have gained minimal traction in the Senate.

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Civil liberties groups and Senator Ron Wyden, an Oregon Democrat, have previously requested information on the extent of U.S. data caught up in the foreign surveillance program.

The Obama administration, however, has said it cannot provide a precise answer and that any estimate would require reviewing communications in a manner that would raise privacy concerns.

In their letter to Clapper, the lawmakers said officials have demonstrated the feasibility of providing an estimate and that any one-time privacy concerns were acceptable in light of the importance of the information.

James Sensenbrenner, Darrell Issa, Jim Jordan, Ted Poe, Jason Chaffetz, and Blake Farenthold were the Republicans to sign the letter. Jerrold Nadler, Zoe Lofgren, Hank Johnson, Ted Deutch, Cedric Richmond, Suzan DelBene, David Cicilline and John Conyers signed for the Democrats. Conyers is the top Democrat on the Judiciary Committee.





(Reporting by Dustin Volz; Editing by Paul Simao)