The director of national intelligence on Saturday stepped up his public defense of a top-secret government data surveillance program as technology companies began privately explaining the mechanics of its use.

The program, code-named PRISM, has enabled national security officials to collect e-mail, videos, documents and other material from at least nine U.S. companies over six years, including Google, Microsoft and Apple, according to documents obtained by The Washington Post.

The disclosures about PRISM have renewed a national debate about the surveillance systems that sprang up after the attacks of Sept. 11, 2001, how broad those systems might be and the extent of their reach into American lives.

In a statement issued Saturday, Director of National Intelligence James R. Clapper Jr. described PRISM as “an internal government computer system used to facilitate the government’s statutorily authorized collection of foreign intelligence information from electronic communication service providers under court supervision.”

“PRISM is not an undisclosed collection or data mining program,” the statement said.

Clapper also said that “the United States Government does not unilaterally obtain information from the servers of U.S. electronic communication service providers. All such information is obtained with FISA Court approval and with the knowledge of the provider based upon a written directive from the Attorney General and the Director of National Intelligence.”

The statement from Clapper is both an affirmation of PRISM and the government’s strongest defense of it since its disclosure by The Post and the Guardian on Thursday. On Wednesday, the Guardian also disclosed secret orders enabling the National Security Agency to obtain data from Verizon about millions of phone calls made from the United States.

Clapper called the disclosures “rushed” and “reckless,” with “inaccuracies” that have left “significant misimpressions.”

“Disclosing information about the specific methods the government uses to collect communications can obviously give our enemies a ‘playbook’ of how to avoid detection,” Clapper said. “Nonetheless, [the law governing PRISM] has proven vital to keeping the nation and our allies safe. It continues to be one of our most important tools for the protection of the nation’s security.”

In responding to the revelations about PRISM, the White House, some lawmakers and company officials have repeatedly suggested that secret court orders are issued every time the NSA or other intelligence agencies seek information under Section 702 of the Foreign Intelligence Surveillance Act. But the orders, which are also secret, serve as one-time blanket approvals for data acquisition and surveillance on selected foreign targets for periods of as long as a year.

The companies have publicly denied any knowledge of PRISM or any system that allows the government to directly query their central servers. But because the program is so highly classified, only a few people at most at each company would legally be allowed to know about PRISM, let alone the details of its operations.