By Nick Bryant BBC correspondent in Washington

Director of Homeland Security Tom Ridge has said cells of Osama Bin Laden's al-Qaeda network are operating in the United States.

It would be very foolhardy to conclude that there were only 19 (al-Qaeda members in the US)

Tom Ridge

In his first interview with British media since he took office last year, Mr Ridge told the BBC there were still glaring weaknesses in security which terrorists could easily exploit.

Although airline safety has improved, he added, the threat from chemical and biological weapons remains a particular concern.

Mr Ridge said America had become much safer since the attacks of 11 September, but he warned that al-Qaeda cells are still waiting to strike.

He also said US law enforcement agencies had already thwarted a number of planned attacks, though he would not say where.

Weak points

Mr Ridge conceded that further terrorist strikes were virtually inevitable.

The administration is preparing for more anthrax attacks

"It would be foolish to conclude - given the fact that at least 19 had made their way months, if not years, before into this country to plan for and prepare for the attacks of 9/11 (11 September) - it would be very foolhardy to conclude that there were only 19," he said.

Much of the government's attention these past twelve months has been focused on airline security.

But Mr Ridge admitted that airports are unlikely to meet targets set by Congress for the screening of all baggage by the end of the year.

Ports are another area of weakness, he said, and a possible entry point for chemical and biological weapons.