BAGHDAD, Iraq--U.N. arms experts paid surprise visits to the homes of two Iraqi scientists in Baghdad on Thursday, and led one of them away after their first foray into private residential quarters in search of evidence of Iraqi weapons of mass destruction.

Also Thursday, Chief U.N. weapons inspector Hans Blix warned Iraq it must cooperate more actively with the weapons inspections if it wants to avoid war.

Blix, in Brussels to meet with European Union officials, said inspections were one way to resolve the dispute but he cautioned: "We feel Iraq must do more than they have done so far in order to make this a credible avenue."

A top adviser of President Saddam Hussein replied that Iraq was ready to resolve any problems with U.N. weapons inspectors.

"All is going well so far. There are some remarks here and there and there are some complaints here and there but we expect to resolve those questions or complaints on Sunday and the next day," Amir al-Saadi told a news conference in Baghdad.

On the eve of the 12th anniversary of the 1991 Gulf War, witnesses said an International Atomic Energy Agency (IAEA) team arrived unannounced in Baghdad's Ghazaliyeh neighborhood, blocked a street and headed to the houses of scientists Faleh Hassan and Shaker al-Jabouri.

Hassan, a physicist, left with the U.N. personnel to an unknown destination after an animated and apparently heated discussion between Iraqi officials and U.N. weapons experts.

Hassan had a box of documents with him as he got into a U.N. car with Dimitri Perricos, a team leader among the U.N. experts searching Baghdad for banned nuclear, chemical and biological weapons and programs to develop them.

"I'm not happy about all of this," Perricos could be heard telling Iraqi officials assigned to accompany the inspectors before driving off with Hassan in a convoy of U.N., Iraqi and journalists' vehicles.

Earlier, the inspectors were seen going through documents at a table set up near Hassan's front door. They had also been engaged in an animated discussion with the Iraqi liaison officers.

Hassan heads al-Razzi State Company which was founded in 1997 by Iraq's Military Industrialisation Commission and employs several people who were involved in Iraq's past nuclear program.

The company was officially involved in laser development and military projects, a U.N. spokesman said when IAEA inspectors visited its facilities in December.

Jabouri is a nuclear scientist who is believed to have been involved in Iraq's past nuclear program.

When the inspectors arrived at the houses, neither man was home, so, respecting Arab traditions, the inspectors waited outside until Iraqi officials brought them back.

The names of the two scientists are believed to be on a list provided by Iraq of 500 scientists involved in past banned weapons programs.

Iraqi officials blasted the inspectors' surprise visits, saying the visits encroached on the human rights of both men, but acknowledged that they did not breach the inspectors' mandate.

The head of a U.N. arms inspection team said on Wednesday his experts would soon interview more Iraqi scientists believed to have been involved in developing nuclear, chemical or biological weapons.

Dimitri Perricos told a news conference in Baghdad that he preferred the interviews to be conducted outside Iraq but the inspectors would not force people to leave Iraq for interviews.

The inspectors have intensified searches since the United States and Britain, who have threatened military force if Iraq does not disarm, supplied them with new intelligence. Iraq denies having any banned weapons programs.

Blix and IAEA director Mohamed ElBaradei said on Thursday to avoid military conflict, Iraq must do more to prove it does not possess chemical, biological or nuclear weapons.

"The message we want to bring to Baghdad is, the situation is very tense and very dangerous and everybody wants to see a verified and credible disarmament of Iraq," Blix said.

ElBaradei, who was in Moscow to meet Russian officials, said: "Iraq should understand that if we continue to report that there are open questions, and we cannot exclude the possibility that they still have some weapons of mass destruction, that will not satisfy the (U.N.) Security Council."

Saadi said Iraq would discuss with both men their claims that Iraq's arms declaration left many questions unanswered.

"All this will be discussed and we will reply officially and hand it over to the Security Council," Saadi said.

He pledged "active cooperation" with the inspectors but said recent calls on Iraq for "proactive cooperation" -- which he said meant that Iraq should hand over banned weapons -- were impossible to meet "because we don't have those weapons."

Iraqi officials said a team of experts from the U.N. Monitoring, Verification and Inspection Commission (UNMOVIC) drove to Mujahideen Khalq, a military base of exiled Iranian rebels, in Karkh some 12 miles from Baghdad.

Another team flew helicopters over the site as the inspection proceeded on the ground. It was the second Mujahideen position to be scrutinized by the experts this week.

But a Mujahideen spokesman in Baghdad denied one of the group's bases was being inspected. He told Reuters that inspectors were searching an Iraqi army position nearby.

Iraq has allowed thousands of well-armed Mujahideen Khalq fighters to be based on its soil since the 1980s. The group claims credit for sporadic guerrilla attacks against Iran.

A Mujahideen spokesman had welcomed the first visit and said the group was ready for more to dispel once and for all Iranian charges that it was hiding banned weapons.

Other inspection teams headed to at least four other sites.

On Wednesday, the experts descended on President Saddam Hussein's main palace compound in Baghdad, inspecting government buildings but staying away from the Iraqi leader's offices.

Iraq called the inspection, the second of one of Saddam's palaces since the experts resumed work on November 27, a provocation and harassment.

Iraq has been under on-again, off-again arms monitoring since the 1991 Gulf War over Kuwait, which erupted on January 17 that year.

Blix and ElBaradei are scheduled to brief the Security Council on January 27 but Blix said he is "almost sure" diplomats will request another report in February.

The inspectors say many gaps must be filled and information analyzed.