A hacker group has claimed to have obtained personal data from 12 million Apple iPhone and iPad users by breaching an FBI computer, raising concerns about government tracking, but the FBI said it never had the data.

The group called AntiSec, linked to the hacking collective known as Anonymous, posted one million Apple user identifiers on Monday purported to be part of a larger group of 12 million obtained from an FBI laptop.

The FBI initially had no comment on the reports, but later in the day issued a statement which cast doubt on the purported data breach, saying it never had the data in question.

"The FBI is aware of published reports alleging that an FBI laptop was compromised and private data regarding Apple UDIDs (unique device identifiers) was exposed," the US Federal Bureau of Investigation said in a statement.

"At this time, there is no evidence indicating that an FBI laptop was compromised or that the FBI either sought or obtained this data."

A tweet from the FBI press office said: "We never had info in question. Bottom Line: TOTALLY FALSE."

Peter Kruse, an e-crime specialist with CSIS Security Group in Denmark, said on Twitter that the leak "is real" and that he confirmed three of his own devices in the leaked data.

"Also notice that they claim to have fullname, addresses, phone numbers etc... Big ouch!" he tweeted.

Apple did not immediately respond to a request for comment.

The fact that some user data was breached prompted a flurry of comments, some suggesting that the government or Apple was implicated in a vast invasion of user privacy.

Aldo Cortesi, a security consultant living in New Zealand, called the incident "a privacy catastrophe."

"The vulnerabilities ranged from de-anonymization, to takeover of the user's gaming social network account, to the ability to completely take over the user's Facebook and Twitter accounts," he said on a blog posting.

One website set up a database to help users determine if their device was on the hacked list of Apple unique device IDs (UDIDs).

Johannes Ullrich of the SANS Internet Storm Center said it was difficult to verify the report.

"There is nothing else in the file that would implicate the FBI. So this data may very well come from another source. But it is not clear who would have a file like this," he told AFP.

Ullrich said it is unclear why the FBI, if the report were true, would have the data.

"The size of the file... would imply a widespread, not a targeted tracking operation, or the file was just kept in case any of the users in the file needs to be tracked," he said.

"The significance of this breach very much hinges on the source, which as far as I know, hasn't been authenticated yet. The data is, however, real based on some of the reports that people do find their own UDID in the file."

In the posting, AntiSec said the original file "contained around 12,000,000 devices" and that "we decided a million would be enough to release."

The group said it "trimmed out other personal data such as full names, cell numbers, addresses, zipcodes, etc."

It said it posted the information to draw attention to Apple's practices, which allow users to be tracked.

"We never liked the concept of UDIDs since the beginning indeed. Really bad decision from Apple," it said.

It added "we have learnt it seems quite clear nobody pays attention if you just come and say 'hey, FBI is using your device details...' FBI IS USING YOUR DEVICE INFO FOR A TRACKING PEOPLE PROJECT OR SOME S---."

The document posted on the website pastebin indicated that the data was obtained in March from the computer of an FBI Cyber Action supervisor through a "vulnerability" in the computer.

Eric Hemmendinger, a security expert with Tata Communications, said that if an FBI computer from a cybersecurity investigator was hacked, it would be "a pretty embarrassing scenario."

Hemmendinger said the FBI's possession of the data would be surprising, but that it should not be a surprise that Apple and its rivals would have detailed information on its users.

"This is yet another indicator that when you start to participate in social networking and applications that Apple and (Google's) Android have propagated, you are the asset that's being leveraged and monetized," he said.

"It's yet another reminder that when you join the social network world, your footprints are not private."