SAN FRANCISCO - Three of the largest US Internet companies called on the US government to provide greater transparency on national security requests on Tuesday, as they sought to distance themselves from reports that portrayed the companies as willing partners in supplying mass user data to security agencies.

Google Inc was the first to go public, releasing an open letter asking the US Department of Justice for permission to disclose the number and scope of data requests each receives from security agencies, including confidential requests made under the Foreign Intelligence Surveillance Act (FISA). Microsoft Corp and Facebook Inc soon followed with similarly worded statements in support of Google.

The three companies, and several others, have come under scrutiny following disclosures last week in The Guardian and Washington Post newspapers of their role in a National Security Agency data collection program named Prism.

Google's letter - which represented the first instance of the Mountain View, California-based company acknowledging that it has received FISA requests - argued that releasing the total number of national security requests would show the company does not give the government "unfettered access" to its users' data.

"Assertions in the press that our compliance with these requests gives the US government unfettered access to our users' data are simply untrue," Google Chief Legal Officer David Drummond wrote in a letter to Attorney General Eric Holder and FBI Director Robert Mueller that was published on Google's public policy blog Tuesday.

"Google's numbers would clearly show that our compliance with these requests falls far short of the claims being made," the letter said.

Google's letter came three days after Director of National Intelligence James Clapper confirmed Prism's existence and described it as an internal computer system that helped the government collect data obtained from Internet companies through FISA requests.

Since 2010, Google has published semiannual transparency reports that reveal the number of data requests it receives from authorities. But the reports have never included requests made under FISA due to their confidential nature.

Following negotiations with the FBI, Google began publishing in March the number of national security letters - confidential requests for the data of domestic users - that it receives from law enforcement officials.

Drummond argued Tuesday that publishing the number of FISA requests would "likewise serve the public interest without harming national security."

Other Internet companies that are typically fierce rivals quickly closed ranks around Google. Sources close to the three companies said the statements were not coordinated.

"Permitting greater transparency on the aggregate volume and scope of national security requests, including FISA orders, would help the community understand and debate these important issues," Microsoft said in an emailed statement.

Leading social network Facebook followed within minutes.

"We would welcome the opportunity to provide a transparency report that allows us to share with those who use Facebook around the world a complete picture of the government requests we receive, and how we respond," Ted Ullyot, Facebook's general counsel, said in an emailed statement.

Twitter's top lawyer Alex Macgillivray also tweeted in support of Google.

"Completely agree with @Google, @SenJeffMerkley & others-we'd like more NSL transparency and @Twitter supports efforts to make that happen," Macgillivray said.