The US government plans to charge foreign students $100 for a tracking system created to prevent possible terrorists from using student visas to enter the country.

The measure was necessitated by reports that the 9/11 hijackers entered the country as "students" and some never attended class, Undersecretary (Homeland Security) Asa Hutchinson said in Washington.

The one-time fee should generate more than $30 million annually for the programme, known as the Student and Exchange Visa Information Service.

Any foreigner enrolled in a school approved by the department must register with the service to enter the US. About 800,000 foreign students enrolled at 8,000 colleges are now in the system, which was set up after 9/11.