Six of the biggest US technology firms are urging Congress to rein in the National Security Agency by requiring more transparency about surveillance and improved privacy protections.

In a letter to a Senate committee, the tech giants applauded the introduction of the USA Freedom Act aimed at ending bulk collection of phone records and improve privacy protection in the Foreign Intelligence Surveillance Court.

"Recent disclosures regarding surveillance activity raise important concerns both in the United States and abroad," said the letter signed by Google, Apple, Microsoft, Facebook, Yahoo and AOL.

The companies, which have failed to win efforts to disclose details of their cooperation with US surveillance programs, said more transparency would "help to counter erroneous reports that we permit intelligence agencies 'direct access' to our companies' servers or that we are participants in a bulk Internet records collection program."

"Transparency is a critical first step to an informed public debate, but it is clear that more needs to be done. Our companies believe that government surveillance practices should also be reformed to include substantial enhancements to privacy protections and appropriate oversight and accountability mechanisms for those programs."

The letter dated Thursday and addressed to the Senate Judiciary Committee came days after a news report said the NSA has tapped into key communications links from Yahoo and Google data centers around the world.

The Washington Post, citing documents obtained from former NSA contractor Edward Snowden and interviews with officials, said the program can collect data at will from hundreds of millions of user accounts, including from Americans.

The report said the program dubbed MUSCULAR, operated jointly with NSA's British counterpart GCHQ, indicated that the agencies can intercept data flows from the fiber-optic cables used by the US Internet giants.

The NSA disputes key details of the report.

The bill proposed by Senator Patrick Leahy and Representatives James Sensenbrenner and John Conyers, with other co-sponsors.

Leahy and Sensenbrenner said in a joint statement they welcomed the wide support for their bill.

"The breadth of support for our bipartisan, bicameral legislation demonstrates that protecting Americans' privacy not only cuts across the party divide, but also addresses concerns raised by the technology industry and other advocates," Leahy and Sensenbrenner said.

"The time is now for serious and meaningful reform. We are committed to working with lawmakers on both sides of the aisle to get this done so we can restore confidence in our intelligence community and protect the privacy rights of our citizens."

The Center for Democracy and Technology, a digital rights advocacy group, meanwhile urged other companies to speak up.

"This is a defining moment and other companies must now step up to support genuine FISA reform or explain to their users why they are not," said Leslie Harris, president and chief executive of CDT.