SAN FRANCISCO (REUTERS) - Several Internet companies have struck an agreement with the United States government to release limited information about the number of surveillance requests they receive, two sources familiar with the discussions told Reuters.

Facebook became the first to release aggregate numbers of requests, saying in a blog post that it received between 9,000 and 10,000 requests for user data in the second half of 2012, covering 18,000 to 19,000 of its users' accounts.

The agreements underscore the pressure imposed on the US government and Internet companies after news leaked last week of a controversial National Security Agency (NSA) programme involving surveillance of foreigners.

The disclosure of that programme triggered concern about the scope and extent of the information-gathering exercise.

Other Internet companies are expected to release numbers of government requests without breaking out how many originate from the NSA, the sources said.

Google, Facebook and Microsoft have publicly urged the US government to allow them to reveal the number and scope of the surveillance requests they receive, including confidential requests made under the Foreign Intelligence Surveillance Act.

Google and Microsoft declined to comment.