The commissioner of the Bureau of Customs and Border Protection, Robert C. Bonner, expressed concern yesterday that Islamic terrorists would gain possession of a nuclear device and transport it to this country, causing widespread fatalities and wreaking economic havoc.

Notwithstanding the Bush administration’s efforts in killing or apprehending major terrorist figures in recent years, Americans “must not let their guard down,” because Al Qaeda operatives pose a continuing threat to the safety, security, and economic wellbeing of America, Mr. Bonner said at the Council on Foreign Relations.

“I worry most about the nuclear threat,” Commissioner Bonner, whose agency is part of the Department of Homeland Security, said. “Al Qaeda wants to get a nuclear device. I’m very concerned that at some point they will get it.”

He added that with more than 9 million containers of cargo coming into the country each year, complete scrutiny of contents remains a problem. He said, however, that more sophisticated forms of detection of weapons of mass destruction had been put in place in several ports abroad from where goods are shipped to America. Some 34 countries were cooperating with the federal government on the “Container Security Initiative,” under which goods being transported to America are required to be examined and registered 24 hours ahead of being shipped. Some 50 more countries were expected to join the program this year, the commissioner said.

His appearance at the council’s headquarters on Park Avenue and East 68th Street came on a day when President Bush named Judge Michael Chertoff as the new secretary of Homeland Security. Mr. Bonner hailed the appointment, praising Mr. Chertoff – a former top official at the Department of Justice – as being “very tough about terrorism.”

Commissioner Bonner’s appearance also came a day after HBO offered Council members a preview of “Dirty War,” a fictionalized film about a nuclear attack on London by Islamic terrorists. The 90-minute film showed in great detail how terrorists transported fissionable material from Turkey to Britain, via Bulgaria. It showed how cells of skilled Islamic operatives assembled so-called dirty bombs. And it showed the dramatic consequences of the explosion of the dirty bomb – hundreds of fatalities, thousands of people contaminated with radio-active material, and millions put in danger of long-term cancer risks.

Mr. Bonner, who attended the screening on Monday evening, said yesterday that such a scenario was entirely plausible. In response to a question by Stephen E. Flynn – the Jeane J. Kirkpatrick senior fellow in national security studies at the council – who presided over yesterday’s talk featuring the commissioner, Mr. Bonner said:

“Global terrorism is likely to dominate the first half of this new century…Al Qaeda and its supporters want to attack the forces of globalization, which leads to economic uplifting. By disrupting global trade, they hope for chaos in America, their main enemy.

“Global trade is now estimated by the Geneva-based World Trade Organization to be around $40 trillion a year. Mr. Flynn, the council fellow, said yesterday that although nearly three years have passed since September 11, 2001 “and despite new security measures, the United States is still dangerously unprepared to prevent or respond to another catastrophic attack on American soil. Faced with this threat, the United States should be operating on a wartime footing here at home.”

Referring to his new book, “America the Vulnerable” (HarperCollins), Mr. Flynn said: “We are living on borrowed time – and squandering it while our most serious vulnerabilities lie ominously exposed.”

Both Mr. Flynn and Commissioner Bonner said yesterday that despite increased public awareness of terrorist threats, America still offers its enemies – in Mr. Flynn’s words – “a vast menu of soft targets: water and food supplies; chemical plants, energy grids and pipelines; bridges, tunnels, and ports; and the millions of cargo containers that carry most of the goods Americans depend upon in their everyday lives.”