Silicon Valley is escalating pressure on President Barack Obama to curb the United States government surveillance programmes that vacuum personal information off the Internet and threaten the technology industry's financial livelihood.



A coalition that includes Google, Apple, Yahoo, Facebook and Microsoft lashed out in an open letter printed on Monday in major newspapers and a new website, http://reformgovernmentsurveillance.com.



The crusade united eight companies that often compete fiercely against each other, but now find themselves banding together to limit the potential damage from revelations about the National Security Agency's snooping on Web surfers.



Twitter, LinkedIn and AOL joined Google, Apple, Yahoo, Facebook and Microsoft in the push for tighter controls over electronic espionage.

As the companies' services and products have become more deeply ingrained in society, they have become integral cogs in the economy.

Their prosperity also provides them with the cash to pay for lobbyists and fund campaign contributions that sway public policy.

PR offensive

Monday's public relations offensive is a by-product of documents leaked over the past six months by former NSA contractor Edward Snowden.

The records reveal that the NSA has been obtaining emails and other personal data from major tech companies under secret court orders for the past five years and scooping up other data through unauthorised hacking into data centres.



Silicon Valley has been fighting back in the courts and in Congress as they seek reforms that would allow them to disclose more information about secret court orders.



Several of the companies are also introducing more encryption technology to shield their users' data from government spies and other prying eyes.



Monday's letter and the new anti-snooping website represent the technology industry's latest salvo in an attempt to counter any perception that they voluntarily give the government access to users' email and other sensitive information.



Although the campaign is ostensibly directed at governments around the world, the US is clearly the main target.



"The balance in many countries has tipped too far in favour of the state and away from the rights of the individual, rights that are enshrined in our Constitution," the letter said.



Civil liberties aren't the only thing at stake. One of the reasons the technology companies have become a rich vein for crime-fighting authorities is that they routinely store vast amounts of personal data as part of their efforts to tailor services and target advertising.



By analysing search requests, Web-surfing habits, social networking posts and even the content of emails, the companies are able to determine, for instance, the type of digital ads to show individual users.



