Admitting that society is too complex to defend against all terrorist threats, a panel of top scientists called for immediate action to identify and repair the weakest links.

The National Academy of Sciences said in a study released Monday that the government needs to tighten control of nuclear materials, assure production of medicine to repel biological attacks, improve transportation security and act to protect energy distribution systems.

Security should be built into system design, the report said, calling for "defense in depth," not just perimeter defense or firewalls.

"Our society is too complex and interconnected to defend against all possible threats," the academy noted. "As some threats are diminished others may arise, terrorists may change their goals and tactics."

Lewis M. Branscomb of Harvard University, co-chairman of the committee that prepared the report, called it a "blueprint for using current technologies and creating new capabilities to reduce the likelihood of terrorist attacks and the severity of their consequences."

"We assume any potential terrorist is looking at all their options, how they might attack us," Branscomb said. "We we need to think about how to derive them of those options."

Rep. Sherwood Boehlert, R-N.Y., chairman of the House Science Committee, welcomed the report as providing an opportunity to focus attention on the need for research in combatting terrorism.

The American Association for the Advancement of Science issued a separate report on combatting terrorism at the same time. Alan I. Leshner, head of that group, stressed the complexity of the problem.

"Yes, terrorism and terrorist acts can come about because of the availability of technology, and you need technology to protect us from terrorist technology," he said.

Leshner cautioned against curbs on scientific communications.

"While protecting ourselves against potential problems, we don't want it to obstruct the future progress in science that we need to advance society," he said.

The massive National Academy report looked at nuclear and radiological threats; possible attacks on human health systems and agriculture; use of toxic chemicals and explosives; the vulnerability of information technology, energy systems, transportation and cities; and the human response to terrorism.

Suggestions for immediate action included:

— Developing improved methods to protect and account for nuclear weapons and other nuclear materials.

— Ensuring the production and distribution of treatments for disease threats.

— Designing and installing in-depth security for transportation, in particular shipping containers and vehicles that carry toxic or flammable materials.

— Improving security for energy distribution systems.

— Developing improved air filtration methods for ventilation systems.

— Ensuring that first responders such as police and fire departments can communicate with one another.

The study was launched after the Sept. 11 attacks, bringing together scientists, engineers, doctors, counterterrorism experts and arms-control specialists from the Academy and its components, the National Academy of Engineering, the Institute of Medicine and the National Research Council.

In its report, the American Association for the Advancement of Science concluded that the nation is poorly prepared to deal with either bioterrorism or attacks on its information systems.

The report by AAAS, the world's largest association of scientists, includes a series of papers looking at the potential hazards of terrorism.

"Bioterrorism is not going to go away," wrote D.A. Henderson, chairman of the Department of Health and Human Services' Council on Public Health Preparedness.

"We are concerned with a comparatively short list of dangerous diseases that would be catastrophic and potentially destabilizing," Henderson said in one of the papers. "They are smallpox, anthrax, plague, tularemia, botulinum toxin and the group of diseases that manifest themselves as hemorrhagic fevers."

In another paper, Eugene H. Spafford, director of the Center for Education and Research in Information Assurance at Purdue University, foresaw a scenario where a computer attack crashes the Internet and half of the nation's telephone network.

"How many would be able to respond effectively without phones or network resources? What kind of panic would ensue?" he asked, calling for more research and investment in increasing security of computer systems.

On the other hand, Eugene B. Skolnikoff, professor of political science at Massachusetts Institute of Technology, urges caution.

"The danger of overreacting is quite real, and in fact, I believe it is already happening," in efforts to restrict some scientific communications, he warned.

Leshner agreed that in fighting terrorism care must be taken not to compromise openness in scientific communications.

"It's a very complex issue, not a simple issue. How do you redefine the relationship between the need to protect security and advance science?"