Canada's principle intelligence agency, the Canadian Security Intelligence Service, says computer hackers are posing a growing threat to national and international security.

In a newly-declassified document, the agency says hacker groups are becoming more sophisticated in their techniques and are increasingly difficult to prevent.

It says military computer systems are particularly vulnerable and says that a group called the Masters of Downloading have reportedly stolen United States military software.

Another organisation, the Milworm Group, is said to have copied sensitive documents from the Bhabha atomic research centre in India.