Yahoo has become the latest tech company to reveal how many requests US law enforcement agencies have made for access to its customers' data.

Online surveillance of citizens by the US government has been in the spotlight since allegations about the National Security Agency's (NSA) PRISM system were published earlier this month.

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It was originally claimed by The Washington Post that PRISM saw the NSA "tapping directly into the central servers of nine leading US internet companies", including Google, Apple, Facebook, and Yahoo.

These claims have since been questioned, and the companies named have denied any involvement . Some of them have been asking US authorities for permission to reveal the actual numbers of requests of user data that they receive from the government.

Yahoo said that between 1 December 2012 and 31 May 2013, it received between 12,000 and 13,000 requests, "inclusive of criminal, Foreign Intelligence Surveillance Act (FISA), and other requests", and said the most common of these requests concerned fraud, homicides, kidnappings, and other criminal investigations.

However, echoing Twitter and Google, Yahoo also called on the US government to allow FISA requests — the most controversial ones because the US government insists in secrecy around them — to be reported separately.

In a blogpost, Yahoo CEO Marissa Mayer and general counsel Ron Bell noted: "Like all companies, Yahoo cannot lawfully break out FISA request numbers at this time because those numbers are classified; however, we strongly urge the federal government to reconsider its stance on this issue."

The company said that later this summer it will publish a "global law enforcement transparency report" which will cover the first half of 2013, and said it plans to refresh the report twice a year.

On Monday, Apple said that between 1 December 2012 and 31 May 2013, it received between 4,000 and 5,000 requests from US law enforcement for customer data. It also claimed that because its iMessage and FaceTime conversations were protected by end-to-end encryption, no-one but the sender and receiver could see or read them.