U.N. inspectors zeroed in on three Iraqi nuclear sites Monday, including one giant complex where Saddam Hussein's scientists had previously tried making material for nuclear bombs.

Also Monday, weapons inspectors paid their first visit to a chlorine plant some 30 miles west of Baghdad. Chlorine can be used to make chemical weapons.

Inspection teams scoured the three nuclear sites near the town of al-Tuwaitha, 15 miles southeast of Baghdad, picking up from where U.N. nuclear agency inspectors left off in 1998, when they left Iraq amid disputes between Baghdad and the United Nations.

Many buildings at the three sites — including the giant al-Tuwaitha nuclear complex — were destroyed in heavy U.S. bombing in the 1991 Gulf War. Through the 1990s, al-Tuwaitha was scrutinized by U.N. nuclear agency inspectors under a postwar U.N. monitoring regime to ensure Iraq did not develop weapons of mass destruction.

Faiz al-Bayrakdar, adviser at the al-Tuwaitha complex, told reporters 16 inspectors visited his site Monday.

U.N. teams want to ensure that Iraqi specialists at al-Tuwaitha and other sites did not resume nuclear weapons research during the four years when no inspectors were in the country. Recent satellite photos show new construction at the plant.

A spokesman for the U.N. inspectors, Hiro Ueki, said inspections also took place Monday at two other nuclear sites — al-Shakyli and al-Qa'qaa — located near al-Tuwaitha.

The al-Shakyli complex had been a storage area where crucial equipment for a nuclear bomb program was discovered during the 1990s. Al-Qa'qaa was involved in working on the final design for a nuclear bomb.

"A detailed inspection was made of al-Shakyli. All buildings were inspected and sampled for the detection of radiological materials," Ueki said.

At al-Qa'qaa, Ueki said inspectors began inventorying known explosive materials from the past nuclear program, which had fallen under control of the International Atomic Energy Agency.

They also inspected several key buildings and outdoor sites within the huge complex. No further details were available.

On Sunday, a presidential science adviser said the three massive reports — totaling more than 12,000 pages — that Baghdad submitted to the United Nations outlined Iraqi efforts to build a nuclear bomb until the Gulf War in 1991.

The adviser, Lt. Gen. Amer al-Saadi, said Iraq no longer has such a program.

Last Wednesday, in their first visit to al-Tuwaitha, specialists of the IAEA — the U.N. nuclear watchdog — spent five hours going "room to room," team leader Jacques Baute reported afterward. But they needed more time to complete their inspection of the complex of more than 100 buildings, he said.

Meanwhile, another group of inspectors paid their first visit to the Falluja II chlorine plant, some 30 miles west of Baghdad, since the inspections resumed last month after a four-year break.

The plant produces chlorine for civilian uses, such as water treatment, as well as caustic soda solution and hydrochloric acid, the plant's director Thair Hazem said.

"It's all very civil production," Hazem said after the U.N. team had spent four hours at the plant, which was destroyed in 1991 and rebuilt three years later.

The team that arrived in six U.N. vehicles split into two groups, Hazem said, one meeting with officials and another searching the compound. Some of the inspectors were seen wearing special protection uniforms during their work.

About 25 new U.N. inspectors arrived on Sunday, doubling the staff and allowing for a rapid expansion of field missions. Over the weekend, the U.N. teams also received the first of an expected eight helicopters that will enable them to range farther afield on their unannounced inspections.

This latest round of inspections began Nov. 27 under a tougher U.N. Security Council resolution aimed at ensuring Saddam no longer possesses weapons of mass destruction.