Britons have been told to be wary of suspect packages and Frankfurt airport was placed on a security alert amid fears of an Islamic extremist mail bomb campaign around Christmas and millennium celebrations. Following warnings in the United States by the FBI, London police urged Britons to be on their guard against unexpected or suspicious-looking packages from unknown addresses overseas. Security at Frankfurt airport was stepped up and a German federal criminal investigation bureau official in Wiesbaden said the organisation was working with U.S. services. The U.S. postal service said there was a possibility that bombs could have been sent by Islamic extremists to addresses there and in Britain. Recent arrests in Jordan and the USA have heightened the public's awareness of a possible threat from Islamic extremist groups, said Deputy Assistant Commissioner Alan Fry of London's Metropolitan Police. There may be groups or individuals who may try to disrupt the celebrations and people should be on their guard against any suspect packages or anyone acting suspiciously. The U.S. has tightened security at entry points since the arrest last week of an Algerian man accused of trying to smuggle explosives into Washington state. Followers of Saudi-born dissident Osama bin Laden were detained in Jordan earlier this month for allegedly planning to attack tourist sites there. The British warning came as the post office grappled with around one million packages daily in the Christmas rush, twice the normal amount.