Homeland Security Secretary Michael Chertoff said on Wednesday that one of the biggest threats to U.S. security may now come from within Europe.

In an interview with BBC radio, Chertoff said that American authorities were becoming increasingly aware of a real risk of Europe becoming a "platform for terrorists".

He said it was important to step up security checks on passengers coming from Europe to the United States.

Chertoff's comments came after reports that British intelligence services are investigating an Islamist Web site which said that a branch of al Qaeda had been established in Britain.