Some American soldiers are missing in the fighting in Iraq and possibly being held as prisoners, Defense Secretary Donald H. Rumsfeld said Sunday. He acknowledged a report of a missing allied aircraft but U.S. defense officials said there was no evidence of that.

The Joint Chiefs of Staff chairman said he thought fewer than 10 soldiers were missing in southern Iraq and that military officials were trying to account for them. "Beyond that, we don't know," Gen. Richard Myers said on Fox News Sunday.

Rumsfeld said he could not provide any information about a missing aircraft. In Baghdad, security officers searched the banks of the Tigris River, apparently looking for one or more pilots who may have bailed out of a downed plane.

Asked what he could say about missing pilots, Rumsfeld replied, "Nothing." He suggested that the search in Baghdad was staged.

"There has been a report of an aircraft missing," the secretary acknowledged on NBC's Meet the Press. "I don't want to speculate because I simply don't know."

Myers said that "as far as we know, all planes are reported to be safe, and so we don't think that's a good report at all. We don't think there are any pilots that have parachuted near the river or anywhere in Iraq."

Rumsfeld said there are some American troops who are missing in Iraq. He noted that under the Geneva Conventions governing prisoners of war, "It's illegal to do things to POWs that are humiliating to those prisoners."

"There are, we believe, there are some American soldiers missing." He said there also could be captured journalists.

Myers said on ABC's This Week that military officials "have nothing to substantiate that claim by the Iraqis, that any pilot has bailed out of his airplane over Baghdad."

Asked whether there were Americans being held prisoner in Iraq, Myers said: "We're still trying to track that one down. ... We're going to have to do some more investigation to determine whether that's true or not."

Rumsfeld characterized the progress of the five-day-old war as excellent, noting that "there are periodic instances when the resistance is quite stiff. ... The fact that there is a firefight, someone ought not to be surprised."

He said the fate of Iraqi President Saddam Hussein remained uncertain. The United States launched air strikes against Baghdad earlier than planned based on intelligence reports that gave U.S. planners hope that they could kill Saddam with an unexpected strike.

"There are reports in Baghdad and in Iraq that he may be dead or that he may be injured," Rumsfeld said. "We'll just have to assume that he is alive and well."

Rumsfeld said if it turns out that Saddam is dead, the United States would not conceal the fact. "My personal view, I would say that the truth is the truth. If he's dead, he's dead."

While coalition forces have moved steadily toward Baghdad, the capital, Myers made clear that troops faced risks ahead.

"This is going to get a lot harder. Anybody that thinks this is going to be quick and easy is wrong. I don't think it's been quick and easy to date," he said.

A senior defense official said coalition forces have yet to come across the six main Republican Guard units, five of which are arrayed around Baghdad and one in Saddam's hometown of Tikrit.

The Pentagon said U.S. troops hunting for banned chemical and biological weapons and Scud missiles have so far come up empty. Rumsfeld said allied forces so far are "not in that business" of searching for weapons of mass destruction inside Iraq because they have been "on the ground 72 hours fighting a war."

The CIA has gathered "massive amounts of information" on weapons of mass destruction and Iraq, Rumsfeld said.

Iraq denies U.S. assertions it has any chemical or biological weapons.

Myers said war planners "have to be prepared for the worst case" -- that Iraqi troops will use such weapons.

"The intelligence, surveillance and reconnaissance is focused looking for those delivery means, the best we can," Myer said.

"It's very hard to see where some of these things are stored, but the delivery means a little more obvious. And so we're going after those delivery means."