WASHINGTON -- The top US intelligence official launched an aggressive defense on Saturday of a secret government data collection program, blasting what he called "reckless disclosures" of a highly classified spy agency project code-named PRISM.

While acknowledging PRISM's existence by name for the first time, Director of National Intelligence James Clapper said it had been mischaracterized by the media. The project is legal, not aimed at US citizens and has thwarted threats against the country, he said.

"Over the last week we have seen reckless disclosures of intelligence community measures used to keep Americans safe," Clapper said in a statement.

He said the surveillance activities reported in the Washington Post and Britain's Guardian newspaper were lawful and conducted under authorities approved by Congress. "Significant misimpressions" have resulted from recent articles, he said.

Clapper's comments were the latest development in an escalating battle over government spying and civil liberties, involving the Obama administration and news organizations that have published details of US data mining efforts.

A fact sheet accompanying the statement discussed in general terms what had been until Thursday an unknown and highly classified program. It made a rare public acknowledgement that US spy agencies obtained data from US telecommunications providers, but defended the practice as legal and regulated by courts.

"The United States Government does not unilaterally obtain information from the servers of US electronic communication service providers. All such information is obtained with FISA Court approval and with the knowledge of the provider," the fact sheet said, referring to the Foreign Intelligence Surveillance Act Court.

PRISM, characterized in news reports as a top-secret National Security Agency program for extracting data from the computers of internet companies, in reality is an "internal government computer system" used to "facilitate" the government's handling of information it collects from service providers, according to the factsheet.

The reports this week said the surveillance program involving internet firms and established under Republican President George W. Bush in 2007, had seen "exponential growth" under President Barack Obama, a Democrat. It said the NSA increasingly relied on PRISM as a source of raw material for daily intelligence reports to the president.

The news reports included PowerPoint slides showing that major Internet companies such as Yahoo, Google , Facebook and a half-dozen others were involved in the program.

Alan Rusbridger, editor-in-chief of the Guardian, had no comment on Clapper's statement. Washington Post spokeswoman Kris Coratti said the company had no comment.

Reaffirms Administration Position

Internet providers have said they knew nothing about any NSA collection program called PRISM and that they have only cooperated with legal government requests for data.

The government can only target someone for internet surveillance if "there is an appropriate, and documented foreign intelligence purpose" for collection, the fact sheet said.

Those purposes include countering terrorism, weapons proliferation and cyber threats, Clapper said in the statement. He did not further explain how those broad targeting guidelines are used in practice.

Previous administration statements in the wake of leaks about the NSA program have not mentioned that it was gathering information related to cyber threats and weapons proliferation.

Clapper strongly reaffirmed the Obama administration's position that US citizens were not "intentionally" targeted by NSA for collection and that the agency's activities were extensively supervised by both Congress and the Courts.

The Guardian published a story on Saturday, based on what it said were more leaked classified NSA documents, about what it described as an internal agency data mining tool created to track the focus of NSA's efforts to collect "metadata" - primitive raw information about message traffic - from around the world.

The newspaper said that a different NSA fact sheet it obtained said that the tool, code-named Boundless Informant, "allows users to select a country on a map and view the metadata volume and select details about the collections against that country."

The Guardian said that documents showed NSA collected "almost 3 billion" pieces of intelligence from US computer networks over a period ending in March 2013. It said the new documents raised questions about what NSA had told Congress about its inability to keep close track on the extent to which it inadvertently collects information about messages sent by Americans.