The National Security Agency's (NSA) collection of telephone records from millions of Verizon customers under a secret court order is an "outrageous breach of Americans' privacy," Sen. Jeff MerkleyJeffrey (Jeff) Alan MerkleyDem senator: If Nielsen doesn't reunite families, 'she should resign' Senate Dems call for Judiciary hearing on Trump's 'zero tolerance' GOP lawmaker compares cages for migrant children to chain-link fences on playgrounds MORE (D-Ore.) said Thursday.

Merkley said the NSA's actions are part of a larger clandestine effort of data collection by the government that needs to be made public.

"This bulk data collection is being done under interpretations of the law that have been kept secret from the public," Merkley continued. "Significant FISA court opinions that determine the scope of our laws should be declassified. Can the [Federal Bureau of Investigations] or the NSA really claim that they need data scooped up on tens of millions of Americans?"

Merkley has long been critical of provisions in the Patriot Act that give intelligence agencies broad powers to collect phone records under orders from the secretive Foreign Service Intelligence Court (FISA). 

A FISA court order obtained by The Guardian shows that the NSA has has been collecting phone records from millions of Verizon business customers since April 25. The highly classified document provides the first concrete evidence that the data-mining practices used during the George W. Bush administration have continued under President Obama.

"I have had significant concerns about the intelligence community over-collecting information about Americans’ telephone calls, emails and other records, and that is why I voted against the reauthorization of the Patriot Act provisions in 2011 and the reauthorization of the FISA Amendments Act just six months ago," Merkley said.

The Obama administration defended the NSA's surveillance on Thursday. A senior administration official called the data collection a “critical tool in protecting the nation from terrorist threats to the United States.”

“It allows counter terrorism personnel to discover whether known or suspected terrorists have been in contact with other persons who may be engaged in terrorist activities, particularly people located inside the United States,” the official said.