The U.S.-led coalition remains "unwavering" in its conviction that there are weapons of mass destruction in Iraq, but tests have proved negative at several suspect sites, a spokesman for Central Command said Tuesday.

Coalition forces are using special equipment to search the Iraqi desert for buried material that might be related to Iraq's chemical, biological and nuclear weapons programs, U.S. Brig. Gen. Vincent Brooks said, noting that entire fighter jets had been found buried. Separately, officials said 11 metal shipping containers were found buried near a Karbala ammunition plant.

"It will take time to uncover things that are deliberately hidden," Brooks said, answering a question about why Iraqi civilians had pointed coalition troops to caches of conventional weapons but had not tipped them off to any weapons of mass destruction.

"We remain convinced that there are weapons of mass destruction inside Iraq and we remain unwavering about that," Brooks said.

U.S. officials have said they do not expect to find much evidence of banned weapons until all parts of Saddam Hussein's former regime are clearly out of power and Iraqis no longer fear the consequences of speaking out.

Brooks said troops were seeking final surrenders from several former regime commanders who have chosen not to fight anymore.

"Military capability throughout Iraq has been destroyed or simply walked away," he said. "But there are still some places where there are leaders that we're in contact with, and we'll seek to take them into our custody, if necessary, or to discuss what the future might be with them."

Coalition forces are working with Iraqis throughout the country to restore order, power and water services, he said. A city council and police force were being formed in Diwaniyah, a leadership council was established in Karbala, and coalition forces were working with local experts to restore power in Nasiriyah, which could help jump-start an adjacent grid in Basra, Brooks said.

U.S. officials recently said they would offer rewards for information about the whereabouts of regime leaders or weapons of mass destruction. Brooks said rewards would also be given for turning in information about terrorists operating in the country, and other sorts of weapons.

He said he was not aware of a "specific price tag" for information about Saddam or other key regime leaders. The United States has offered a $25 million reward for information leading to the capture of Osama bin Laden, leader of the al-Qaida terror network.

Brooks was also asked why U.S. forces had failed to guard the Iraqi National Museum, which was looted of priceless antiquities when Baghdad descended into chaos last week. He said troops were heavily occupied by combat when they entered the capital.

"I don't think anyone anticipated that the riches of Iraq would be looted by the people of Iraq," he said.

The museum's director of antiquities said that Marines had promised several days ago to protect what was left of the collection, no help came.