"There is going to be an attack. Whether it is in western Europe, the US, Africa, Asia or wherever, you have got to anticipate that there is going to be a bioterrorism attack and the only way to defend yourself is by getting prepared," said Tommy Thompson, health secretary.

In an interview with the Financial Times, he said the wave of arrests in Britain, France, Spain and Italy, and the uncovering of terrorists' attempts to make the deadly poison ricin, made the issue more urgent. Countries were not doing enough, he said.

Mr Thompson met health ministers and officials from the G20, the leading industrialised and developing countries, at the World Economic Forum in Davos, Switzerland, on Sunday night to try to step up international efforts in research and vaccine-sharing and agree mutual assistance pacts to support a country that was attacked.

Since the anthrax panic of 2001, the US has increased measures against bioterrorism. Last year it spent $1.1bn (£700m), and is spending $4.5bn this year with a similar amount planned next.

It has purchased enough smallpox vaccine for the entire population, stockpiled antibiotics and other drugs at 12 sites within seven hours' reach of any community, and is seeking new vaccines for botulism, haemorrhagic fever viruses, plague and anthrax.

Mr Thompson said other countries were "light years" ahead of where they were a year ago in preparedness, but still had a long way to go. They too needed to purchase smallpox vaccines and develop comprehensive plans to gear up their health infrastructure to deal with an attack.

"The preparation is difficult, hard, expensive, but the lack of a comprehensive plan can really damage your economy as well as your population. These poisons have the capacity of killing thousands of people," he said.

An attack could come in the form of "a bioterrorism agent, a chemical dispersal or a radiological one". Smallpox was the most devastating threat because of its contagious nature, he said, but poisoning of food with ricin was a great concern.

Although attention has focused on western Europe as a potential target, terrorists could strike anywhere. "Every country is at risk. You do not know whether a suicide bomber is going to be willing to sacrifice his or her own life to damage somebody else's. It is impossible to defend against, so you have got to be prepared," he said.

Mr Thompson said the US was "by far the world's leader" in the fight against bioterrorism but was still in the process of implementing its plans. "Unless you are prepared to react, your community of citizens are going to be severely impacted by deaths and severe illnesses."