US tech giant Apple has revealed it received between 4,000 and 5,000 data requests in six months from US authorities, days after Facebook and Microsoft released similar information. Apple, Facebook, Microsoft and several other top internet and technology companies have come under heightened scrutiny since word leaked of a vast, covert web surveillance program that US authorities insist targets only foreign terror suspects and has helped thwart attacks. On its website yesterday, Apple said between December 1 last year and May 31, federal, state and local law enforcement had requested customer information up to 5,000 times, related to between 9,000 and 10,000 accounts or devices. Most commonly, these requests were related to criminal investigations, searches for missing children or patients with Alzheimer's disease, or hoping to prevent a suicide, Apple said. But the iPhone maker said it works vigorously to protect the privacy of its users and only provides information by court order. "Regardless of the circumstances, our legal team conducts an evaluation of each request and, only if appropriate, we retrieve and deliver the narrowest possible set of information to the authorities," it said, noting that sometimes the requests were denied altogether. Apple also specified certain types of communications are protected, such as FaceTime and iMessage conversations, which are "protected by encryption". "Apple cannot decrypt that data," it said. "Similarly, we do not store data related to customers' location, map searches or Siri requests in any identifiable form." Facebook said on Friday it had received 9,000 to 10,000 requests for user data affecting 18,000 to 19,000 accounts in the second half of last year, while Microsoft said it had received 6,000 to 7,000 requests affecting 31,000 to 32,000 accounts in the same period. The firms denied claims the National Security Agency could directly access their servers.