Disclosure of the massive surveillance of phone records and internet communications risks “long-lasting and irreversible harm” to US national security, the director of national intelligence says.

Late on Thursday night US time James Clapper issued a bullet-point defence of the surveillance programs disclosed by the Guardian and the Washington Post, saying they contained “numerous safeguards that protect privacy and civil liberties”. To correct the “misleading impression left in the article” – apparently a reference to the Guardian’s original story – Clapper said he approved the declassification of his defence of the National Security Agency’s collection of every phone record from millions of Verizon customers.

“There is a robust legal regime in place governing all activities conducted pursuant to the Foreign Intelligence Surveillance Act,” Clapper wrote, “which ensures that those activities comply with the Constitution and laws and appropriately protect privacy and civil liberties. The program at issue here is conducted under authority granted by Congress and is authorized by the Foreign Intelligence Surveillance Court (FISC). By statute, the Court is empowered to determine the legality of the program."

Clapper attacked the disclosures by the Guardian and the Washington Post as "reprehensible" for risking "important protections for the security of Americans".

A judge for Fisa Court, as the surveillance body is known, reviewed and approved the surveillance. But critics have pointed out that the Fisa Court has almost never, in its 35-year history, rejected a US surveillance request – a perception of docility that prompted its presiding judge, Reggie Walton, to defend the court’s integrity in a statement to the Guardian on Thursday.

Clapper said the Fisa Court had established procedures preventing the government “indiscriminately sifting” through the collected phone records. “The court only allows the data to be queried when there is a reasonable suspicion, based on specific facts, that the particular basis for the query is associated with a foreign terrorist organisation,” Clapper said. “Only a small fraction of the records are ever reviewed” by “specifically cleared counterterrorism personnel”.

At the same time, Clapper said national security required the NSA to collect all the Verizon subscriber data, even if not all the data would be analysed, and regardless of any evidence to link the phone records to crime, foreign espionage or terrorism. On Thursday, the Wall Street Journal reported that other telecoms received similar orders from the government for the subscriber data.

“The collection is broad in scope,” Clapper wrote, “because more narrow collection would limit our ability to protect the nation from terrorist threats to the United States, as it may assist counterterrorism personnel to discover whether known or suspected terrorists have been in contact with other persons who may be engaged in terrorist activities.”

Yet the collection does not need to be tied to terrorism to occur – something that alarmed one Democrat senator, Jeff Merkley. He told the Guardian on Thursday that the sweeping “barn-door” collection appeared to violate the provision of the Patriot Act purportedly authorising it.

“We can't really propose changes to the law unless we know what the words mean as interpreted by the court,” Merkley said.

Clapper reiterated a point the Obama administration made on Thursday in its response to the Guardian’s story: the NSA’s dragnet of Verizon phone records, which the Fisa Court authorised until 19 July, does not include the “content of any communications or the identity of any subscriber”. Yet the so-called “metadata” – phone numbers, duration of calls – can be combined with publicly available information to easily determine subscriber identity. And a second NSA surveillance effort, disclosed by the Guardian on Thursday and codenamed PRISM, collects the content of communications provided through Google, Facebook, Microsoft, Apple and five other large internet companies.

Clapper came under criticism on Thursday for statements to Democrat senator Ron Wyden that appeared to be contradicted by the revelations of the surveillance programs. Asked in March whether “millions” of Americans had “any kind of [their] data” collected by the US government, Clapper replied: “Not wittingly. There are cases where they could inadvertently perhaps collect, but not wittingly."

He has denied misleading Congress, but Clapper’s statement on Thursday suggested the collection of Americans’ phone records was deliberate, methodical and institutionalised.

“Discussing programs like this publicly,” Clapper concluded, “will have an impact on the behavior of our adversaries and make it more difficult for us to understand their intentions.”