A suicide bomber in central Iraq has killed four American soldiers as coalition warplanes continue to pound Baghdad.

U.S. military officials say a taxi stopped at a checkpoint on the highway north of Najaf and waved for help. When soldiers approached the car exploded.

VOA's Alisha Ryu, traveling with the 3rd U.S. Army Infantry Division in Central Iraq, says coalition forces are now tightening security all over the country.

"This is the first time that there has been a car bombing, and a suicide car bombing at that," she said. "Previous incidents have all been very limited to pickup trucks that have been harassing U.S. soldiers, gun-mounted pickup trucks, those kinds of situations with small arms fire. This is the first time that they have actually resorted to suicide attacks. And, I can tell you that security has been massively heightened all over Iraq."

Coalition leaders have charged that Iraqi irregular forces are dressing as civilians and then firing on U.S. and British soldiers.

Iraq's information minister, Mohammed Said al-Sahhaf, called the charges cheap propaganda, saying Iraqi civilians are simply defending themselves.

"There is no Iraqi military disguising himself," he said. "Those are Iraqi civilians fighting against those mercenaries. Our people [are] fighting those mercenaries and invaders and they will be eliminated gradually."

Coalition warplanes are continuing to pound the Iraqi capital throughout the day.

Iraq says 68 people have been killed in Baghdad from coalition air strikes in the past 24 hours.

U.S. officials say they are investigating reports that a bomb killed dozens of people late Friday at a crowded market in the capital.

In southern Iraq, U.S. warplanes bombed a building near Basra where 200 paramilitary fighters were meeting. Coalition officials say they believe all the Iraqis were killed.

During a news conference in Qatar, U.S. Air Force Major General Gene Victor Renuart, the coalition's director of operations, admits several U.S. Tomahawk cruise missiles fired towards Iraq have gone astray.

"In the case of Saudi Arabia, we did have a number of T-LAM [Tomahawk] missiles that were reported down in their territory," the general said.

General Renuart says U.S. forces have agreed to limit flights of the missiles over Saudi territory, a move he says that will not have an impact on coalition operations.

Early Saturday an Iraqi missile exploded near a shopping mall in Kuwait City, slightly wounding two people. In the past week, 13 missiles have been fired at Kuwait, but this was the first to reach the capital. Information Minister Ahmed Fahd al-Sabah says his country is now facing a dangerous threat.

"Nobody was thinking about a missile in Kuwait. Now, I think everybody can see it with their eyes, and that means Kuwait is in a very dangerous situation, because it's in the range of those kind of missiles," he said.

In his weekly radio address President Bush said coalition troops are still making a "steady advance" toward Baghdad, and coalition forces are now within 80 kilometers of the Iraqi capital.

Mr. Bush says it is not known how long the war will last, but the outcome remains certain.

"We are now fighting the most desperate units of the dictator's army," the president said. " The fighting is fierce and we do not know its duration, yet we know the outcome of this battle: The Iraqi regime will be disarmed and removed from power. Iraq will be free."

President Bush says fighting is fierce between Iraqi and coalition forces. The president called recent atrocities by Iraqi forces "the cruel nature of a dying regime."

The president again promised that war criminals will be brought to justice and punished severely after the war.

Meanwhile, the latest survey of U.S. public opinion (Washington Post-ABC News) released Saturday shows that 75 percent of Americans support the war against Iraq.