Tech giants including Apple, Facebook and Google called for substantial reforms to the US government's surveillance programmes Thursday in a letter to the Senate judiciary committee.In the wake of more revelations about the lengths to which the National Security Agency has gone to intercept data, the companies have called for more transparency and "substantial enhancements to privacy protections and appropriate oversight and accountability mechanisms for those programs."The letter, also signed by AOL, Microsoft and Yahoo, follows the release of more documents obtained by former NSA contractor Edward Snowden that reveal the US authorities were secretly tapping in to the tech firm's main communications links.The letter "applauds" the USA Freedom Act, a bill sponsored by Democrat senator Patrick Leahy and Republican congressman James Sensenbrenner that would end the bulk collection of data from millions of Americans and set up a privacy advocate to monitor the Fisa court, which oversees the NSA's US activities.