Personal data belonging to more than 45,000 employees and retirees of the US Federal Aviation Administration (FAA) was stolen after hackers broke into a computer server, the agency has admitted.

Two of the 48 files on the server contained personal data for those on the agency's employee rolls for the first week of February 2006, the FAA said. Those affected will be notified by letter, and law enforcement has been contacted.

The breach did not affect air traffic control or other operational systems, the FAA said.

"The FAA is moving quickly to prevent any similar incidents and has identified immediate steps as well as longer-term measures to further protect personal information," according to a statement.

The FAA is the latest US government agency to be affected by a data breach, incidents of which have been increasing by the year as hackers successfully target servers with weak security. Other causes include the loss or theft of laptops containing unencrypted information.

The Identity Theft Resource Center (ITRC) said in January that more than 35 million data records were breached in 2008 in the US, a record number.

In one of the most serious incidents affecting the US government, the Department of Veterans Affairs reported that a laptop and hard drive containing personal information of 26.5 million military veterans and their spouses was stolen from an employee's home in May 2006.