Director of National Intelligence James Clapper did not disclose the number of times the NSA had searched for Americans’ communications without a warrant under a program authorized under Section 702 of the FISA Amendments Act. (Pablo Martinez Monsivais/AP)

Director of National Intelligence James R. Clapper Jr. acknowledged that the National Security Agency has searched for Americans’ communications without warrants in massive databases that gather e-mails and phone calls of foreign targets.

Although recently declassified documents made clear that the NSA had conducted such searches, no senior intelligence official had previously acknowledged the practice. Clapper did so in a letter to Sen. Ron Wyden released Tuesday.

Clapper did not disclose the number of times the NSA had searched for Americans’ communications without a warrant as part of a program authorized under Section 702 of the FISA Amendments Act of 2008.

The program targets foreigners abroad for surveillance but captures potentially large volumes of the communications of Americans in contact with those foreigners. According to declassified court documents, the NSA harvests about 250 million Internet communications a year under the program.

The program, which does not require individual warrants for targets, is being challenged in federal court as unconstitutional. The government contends it does not need a warrant to perform queries within a database of communications it has collected lawfully.

“This is unacceptable,” Wyden (D-Ore.) and Sen. Mark Udall (D-Colo.) said in a statement. “It raises serious constitutional questions, and poses a real threat to the privacy rights of law-abiding Americans.”

The senators have co-sponsored legislation that would require the government to seek a warrant before querying the Section 702 database for Americans’ data.

Last month, Robert S. Litt, the general counsel in the Office of the Director of National Intelligence, appeared to suggest that the number of queries involving Americans’ data was high enough to make court approval for each query impractical.

“The number of times that we query the 702 database for information is considerably larger” than the number of times queries are made of the NSA’s telephone records database assembled under a program to search for clues to terrorist networks, Litt told the Privacy and Civil Liberties Oversight Board, an independent watchdog.

The NSA queried the phone records database using numbers suspected of being linked to terrorism 288 times in 2012.

Beginning in February, the NSA has had to seek court approval for each phone number it wants to query in that database.

“I suspect that the [Foreign Intelligence Surveillance Court] would be extremely unhappy if they were required to approve every such query” made within the 702 collection, Litt said.

Asked after the hearing to confirm that Litt was speaking about queries involving Americans’ data, ODNI spokesman Michael Birmingham said in an e-mail: “Bob said he was speaking about the programs generally, and did not mean to imply anything else since he did not specifically break out US Persons in his response.”