Defense Secretary Donald H. Rumsfeld has ordered an investigation into the mistaken bombing in Afghanistan that killed three U.S. soldiers.

The Army Green Berets were killed and 20 other Americans were wounded Wednesday when an Air Force B-52 dropped a one-ton bomb near them. The soldiers had called in the airstrike to hit Taliban forces they were fighting near the militia's southern stronghold of Kandahar.

Five anti-Taliban Afghan fighters also were killed and another 18 were wounded.

The Pentagon identified those killed as Master Sgt. Jefferson Donald Davis, 39, of Watauga, Tenn.; Sgt. 1st Class Daniel Petithory, 32, of Cheshire, Mass.; and Staff Sgt. Brian Cody Prosser, 28, of Fraizer Park, Calif. All were members of the Army's 3rd Battalion, 5th Special Forces Group, stationed at Fort Campbell, Ky.

All but three of the wounded were evacuated from the scene, first to a U.S. Marine base south of Kandahar and then out of Afghanistan. The injuries to the Americans taken outside of Afghanistan ``vary from moderate to severe,'' a statement from U.S. Central Command said.

Eighteen Afghan anti-Taliban fighters were being treated on U.S. Navy ships in the Arabian Sea, the statement said. Eight were on the USS Peleliu and 10 aboard the USS Bataan.

Pentagon officials said they could not immediately explain what went wrong in the deadliest ``friendly fire'' accident of the war. Whatever the cause, it illustrated the danger to U.S. forces calling in airstrikes.

``This is one of the potentially most hazardous type of missions that we use as a military tactic,'' said Rear Adm. John Stufflebeem, deputy director of operations for the Joint Chiefs of Staff.

Hamid Karzai, the southern Pashtun leader and newly designated head of the provisional government in Afghanistan, was in the area where the bomb landed but was not seriously wounded, Pentagon officials said. Karzai told reports early Thursday he received minor injuries of the face and head from flying glass.

``I, along with the rest of America, grieve for the loss of life in Afghanistan,'' President Bush said during an Oval Office appearance. ``I want the families to know that they died for a noble and just cause.''

The Pentagon initially reported two were killed. A third died en route to a hospital.

The deaths bring to four the number of Americans killed inside Afghanistan in the two-month war. CIA officer Johnny ``Mike'' Spann was killed Nov. 25 in a prison uprising while questioning forces captured in the fighting.

The United States is focusing its bombing on Kandahar and the mountainous area near the Khyber Pass south of Jalalabad, where it is believed Usama bin Laden and his top lieutenants are hiding in a complex of caves and tunnels.

Several hundred Green Berets and other U.S. special operations troops are in Afghanistan to assist the Afghan opposition forces. The Pentagon has credited them with helping turn the tide against the Taliban in northern Afghanistan last month by enabling more precise and effective U.S. bombing. In the south, they are helping a collection of anti-Taliban forces in a fierce fight for Kandahar.

U.S. officials familiar with the evolving battle for Kandahar said it appeared that the Afghan opposition forces were too few to initiate a final, climactic assault on the Taliban and al-Qaida holdouts inside the city. Nonetheless it remains the Pentagon's strategy to let the opposition forces take the lead on the ground.

Victoria Clarke, Rumsfeld's spokeswoman, said the U.S. casualties Wednesday were a reminder of military members' willingness to risk their lives in fighting an unconventional war triggered by the Sept. 11 attacks.

``Every casualty rests at the feet of the Al Qaeda and the Taliban,'' she said.

Stufflebeem said it would take a few days to sort out what happened.

A forward air controller among the U.S. troops involved in the incident called for close air support and an Air Force B-52 bomber responded by launching a bomb known as a Joint Direct Attack Munition, he said.

The bomb is guided by a satellite navigation system and is considered one of the most accurate weapons in the U.S. arsenal. It was used for the first time in combat in Kosovo in 1999.

Stufflebeem and other Pentagon officials said investigators will try to determine whether the bomb missed the intended target because of mechanical or human error.