KABUL, Afghanistan — A suicide car bomber struck a convoy of U.S. military vehicles in downtown Kabul today, killing at least 16 people, including two American soldiers, and wounding 29 others, officials and witnesses said.

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, one of the worst in Kabul in recent years, 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.

Afghanistan is facing its deadliest spate of violence since U.S.-led forces toppled the hard-line Taliban regime for hosting Osama bin Laden.

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 in Poland this weekend.