Iraq has recently stepped up attempts to import industrial equipment that could be used to enrich uranium for use in nuclear weapons, a U.S. intelligence official said Saturday.

Several equipment shipments destined for Iraq have been stopped, the official said, declining to say by whom or where. They included a precisely made kind of metal tube that can be used in uranium-enrichment programs, the official said.

It is unclear whether any other shipments got through. The official said Iraq's efforts to obtain the tubes have take place in recent months, declining to provide more details to protect American intelligence sources.

U.S. intelligence officials, however, do not believe Iraq has obtained any enriched uranium or plutonium. Without those materials, Iraqi President Saddam Hussein cannot build a nuclear weapon.

However, officials described the shipments as evidence that Saddam has been trying to revitalize his nuclear weapons program -- and also evidence he does not have a nuclear weapon.

Saddam's weapons programs are the Bush administration's main justification for threatening war against Iraq.

Over the years, Iraq has made numerous attempts to import various kinds of equipment that could be used in nuclear, biological or chemical weapons' research and manufacturing programs.

Some of this equipment, however, also has benign uses in manufacturing, medicine or other industries, so U.S. intelligence officials aren't always certain whether a particular piece of equipment is destined for Saddam's weapons program.

The New York Times and The Washington Times both have reported on Iraq attempts to buy metal tubes that could be used to enrich uranium.

Information on Iraq's programs has been spotty since Saddam expelled U.N. weapons inspectors in 1998. Iraq is believed to have kept some chemical and biological weapons, such as mustard gas and anthrax, as well as a few Scud missiles.

But unless Saddam provides these weapons to terrorists, they are primarily a threat to Israel and U.S. troops in the field, not American cities.