When Pakistani nuclear scientist Abdul Qadeer Khan wanted to ship nuclear-related materials to spots like Libya or Iran, his customers had little to fear. The contraband goods, hidden amidst the huge number of massive shipping containers that move every day through the port of Dubai in the United Arab Emirates, easily escaped detection.

Former Deputy Assistant Secretary of Defense Peter Brookes, now a senior fellow at the Heritage Foundation, said Mr. Khan's actions have underscored the vulnerability of high-volume seaports to high-tech smugglers. "What we really have learned from this - not focusing just on Dubai," he said," [is] there are a lot of places in the world where these sorts of materials, this sort of contraband, can pass through."

And if a simple smuggler can so easily move his wares through a port, it is even more frightening to contemplate what a determined terrorist might be able to accomplish.

Each day in ports around the world, thousands of huge metal cargo containers are loaded on and off of ships. That, say analysts, makes the odds heavily in the smuggler's favor. It is simply impossible for customs agents to thoroughly search every container.

A thorough scrutiny of one container may take up to eight hours. In many places, laws may be lax and customs officials corrupt. As Mr. Brookes pointed out, in a high-density port, containers can easily be moved in and out of a port with false documentation about the contents and destination.

"Sometimes one or two percent of things are inspected," said Mr. Brookes, "and the people, the smugglers who move these sorts of things, don't write on the box 'centrifuge parts for nuclear weapons' or 'for nuclear power plants.' They disguise these sort of things. There is a lot of documentation that is false. Things are often mislabeled. They are not actually what they say they are."

Experts say the status of Dubai as an open port and a freewheeling trade zone, and its location straddling the Persian Gulf and the Indian Ocean, make it especially attractive to smugglers, particularly those operating out of South Asia and the Middle East. According to news accounts, everything from Western cigarettes to pirated computers flows through Dubai. It has figured in several nuclear arms diversion cases in the 1980s and 1990s, as well as figuring prominently in the Khan case. Last month President Bush singled out SMB Computers, a Dubai-based company, as a front for the Khan network.

Gary Milhollin, director of the Wisconsin Project on Nuclear Arms Control in Washington, called Dubai a haven of illicit activity, and sharply criticized authorities there for what he says is a lack of cooperation in cracking down on the nuclear proliferators operating through Dubai. But it is not, he said, the only such hub.

"Dubai is by far the biggest offender, although there are other countries that have come to attention as re-transfer points," said Mr. Milhollin. "One of them is Hong Kong. If everything in Hong Kong that was supposed to go to Hong Kong were in Hong Kong, the place would sink. There's not enough room for everything."

Analysts cite Rotterdam, Hamburg, Singapore, and New York as other worrisome ports of entry.