The blast near the U.S. Embassy came as NATO chiefs appealed for member nations to send reinforcements to combat resurgent Taliban militants fanning the deadliest violence in five years. A top British general said the fighting in volatile southern Afghanistan was now more ferocious than in Iraq.

The bomb blew pieces of an American Humvee and U.S. uniforms into trees, which were set ablaze by the explosion. The blast shattered windows throughout downtown, and a cloud of brown smoke climbed hundreds of feet into the sky.

The bombing came three days ahead of the fifth anniversary of the Sept. 11 attacks and as Afghans remembered Ahmad Shah Massood, the fabled Northern Alliance commander who fought Soviet forces and the Taliban and was assassinated by suspected al-Qaida operatives posing as journalists on Sept. 9, 2001.

The Kabul blast went off about 50 yards from the landmark Massood Square, which leads to the main gate of the heavily fortified U.S. Embassy compound. It dug a 6-foot-wide crater and left body parts, Muslim prayer caps, floppy khaki-colored military hats and shoes scattered on the ground.

President Hamid Karzai condemned the attack, saying, "Today's heinous act of terrorism is against the values of Islam and humanity."

A witness, Najibullah Faizi, said he saw a blue Toyota Corolla driven by a young, heavyset man speed past another car on the inside lane before ramming one of two U.S. Humvees in a convoy.

"I fell to the ground after the blast. American soldiers started shooting at another car nearby. There was smoke and flames everywhere," Faizi, 25, told reporters.

Sixteen people were killed and 29 were wounded, said Ali Shah Paktiawal, criminal director of the Kabul police. Two American soldiers in the vehicle were among those killed and two were among the wounded, said U.S. military spokeswoman Lt. Tamara Lawrence. The attacker also died.

Among the victims was an elderly woman who had been sitting with her granddaughter in a small yard outside a Soviet-era apartment building.

"My mother just went to the park for some fresh air with my daughter when the explosion happened," said the woman's son, Farid Wahidi, 40. "Shrapnel hit her in the chest and killed her."

An Associated Press reporter saw the bodies of two coalition soldiers lying yards from the Humvee. U.S. troops stood guard around the bodies, one of which was slumped in the gutter, the other covered by a plastic sheet.

Dozens of U.S. and British soldiers cordoned off the scene as investigators sifted through the wreckage of the charred military vehicle.

Soldiers retrieved body parts, apparently from the suicide bomber, and placed them into plastic bags for further investigation.

Afghanistan is facing its deadliest spate of violence since U.S.-led forces toppled the hard-line Taliban regime for hosting Osama bin Laden. Hundreds on both sides have been killed each month this year.

A roadside bomb hit an Italian military convoy in western Farah province Friday, wounding four troops, one seriously, NATO and the Italian Defense Ministry said.

Some 20,000 NATO soldiers and a similar number of U.S. forces are in Afghanistan trying to crush an emboldened Taliban insurgency. The heaviest fighting takes place across vast desert plains in southern Helmand and Kandahar provinces, also center of the country's massive opium trade.

"The fighting is extraordinarily intense. The intensity and ferocity of the fighting is far greater than in Iraq on a daily basis," Brig. Ed Butler, the commander of British Forces in Afghanistan, told British ITV news.

He echoed NATO commander Gen. James L. Jones' call Thursday for more troops. Jones, who said the next few weeks would be decisive in the fight against militants, was expected to press officials from the 26 NATO member states for more soldiers and air support at talks being held in Poland on Friday and Saturday.

Butler said more soldiers would allow operations to be carried out faster. "It will continue to be tough and we will continue to take more casualties, but morale is extraordinarily high," he said.

Also Friday, a would-be suicide attacker killed only himself when his bomb-packed car exploded prematurely in Kandahar, said police official Rehmat Ali.

The car was parked on the main road to the Kandahar Airfield, where NATO vehicles, Afghan security forces and government officials regularly pass. None were in the area at the time of the blast.

Afghan security forces, meanwhile, found four bombs near a northern Kabul high school, defusing two and safely detonating the others, said police official Mohammed Arif.

About 70 Taliban fighters fired rockets at a district government headquarters in the central province of Wardak early Friday before police repelled them, said provincial police chief Mahboobullah Amiri.

Eight Taliban were killed and four wounded according to witnesses, Amiri said, but police had retrieved no bodies. One policeman was lightly wounded, and eight militants were arrested.

NATO forces launched airstrikes and artillery and mortar barrages on Taliban positions in Kandahar's Panjwayi district overnight, inflicting an unspecified number of Taliban casualties, said Maj. Scott Lundy, a NATO spokesman. No NATO or Afghan forces were hurt.

Lundy said NATO would press on with Operation Medusa, which began Saturday in Panjwayi, until it had "removed" all the Taliban militants. NATO says it has killed more than 270 insurgents since the offensive began and that hundreds more are massed in the district, west of Kandahar.

Copyright 2006 Associated Press. All rights reserved.