With the clock ticking inexorably toward midnight, the chief U.N. weapons inspectors were heading to Baghdad Thursday in a last-ditch effort to persuade Iraq to prove, finally, that it has disarmed, and avoid a war in the Persian Gulf.

In the wake of Secretary of State Colin Powell's presentation to the U.N. on Wednesday, Hans Blix and Mohamed ElBaradei know they face an uphill battle.

Blix told Fox News while en route to London that he was impressed by the audio intercepts that Powell presented to the Security Council, saying they "were something I had not seen before."

But Blix added that "we have to look at everything with a critical eye, and that goes also for [information provided by] the U.S.

"I'm not casting doubts, necessarily, but do you get the impression that Iraq has weapons of mass destruction, or that they are concealing something? I think the major impression is the latter."

Asked if he will discuss Powell's information with the Iraqis when he gets to Baghdad this weekend, Blix said:

"I don't think we have to. They heard Powell's briefing, and we have an agenda and we will follow that."

Blix said he didn't get particularly angry when he heard intercepts of Iraqi officials discussing how to keep inspectors from discovering banned materials.

"Well, we had assumed that we were being observed," he said. "But if they want to claim that they have no weapons, it would be better if they were more open instead of making us feel that we are being observed and led by the nose."

Meanwhile, ElBaradei, the chief nuclear inspector, demanded that Iraq show a "drastic change" in its cooperation with the hunt for banned weapons of mass destruction during their talks this weekend.

"Iraq is not cooperating fully; they need to show drastic change in terms of cooperation," ElBaradei said in London after he and Blix met British Prime Minister Tony Blair.

Blix and ElBaradei will be in Baghdad in advance of their next report to the Security Council on Feb. 14. Blix told the council Jan. 27 that Iraq has yet to show a "genuine" desire to disarm, an assertion that prompted the Iraqis to pledge more cooperation.

"We need to show progress in our report," ElBaradei said Thursday. "Our mission in Baghdad this weekend is crucial. We hope we will secure full, 100 percent cooperation on the part of Iraq."

Powell said he would be watching the trip closely to see "whether they bring back anything of use for Security Council deliberations" next week.

In Iraq, weapons inspectors visited a military engineering academy, a brewery and an army complex that includes a missile site west of Baghdad. They also paid surprise visits to the water and sewage authority and the laboratories of a health care complex in Baghdad. Inspectors returned to a storage facility belonging to Al-Tuwaitha, the center of Iraq's former atomic program south of Baghdad, which they had inspected the day before.

Presidential adviser Lt. Gen. Amir al-Saadi, speaking at a news conference in Baghdad on Wednesday night, suggested that telephone monitored Iraqi conversations played by Powell were fabricated, that defector informants were unreliable, and that the satellite photographs he displayed "proved nothing."

"What we heard today was for the general public and mainly the uninformed, in order to influence their opinion and to commit the aggression on Iraq," al-Saadi said.

"This was a typical American show, complete with stunts and special effects," he said.

Most U.S. allies want more time for U.N. weapons inspectors to do their work. But President Bush has signaled his impatience with the inspections process and pledged to disarm Iraq forcibly if it does not immediately comply with U.N. resolutions -- with or without its allies. The United States already has some 100,000 troops amassed in the Gulf region and the number could rise to 180,000 in a matter of weeks.

Of the 15 Council members, only the United States and Britain have offered unwavering support for forcibly disarming Saddam.

Iraqi Ambassador Mohammed Al-Douri, who took part in Wednesday's Security Council session, said he was satisfied that most of the council -- including France, Germany and Syria -- favored giving inspectors more time to do their work rather than rushing to military action.

"It's obvious that Mr. Powell's remarks did not achieve the results the U.S. administration intended," Al-Douri said.

The Associated Press contributed to this report.