Three American contractors were among 12 people killed Saturday in a suicide car bomb attack in the heart of the Afghanistan capital near a hospital, officials said.

They were riding in a convoy of NATO armored vehicles traveling through a crowded neighborhood in Kabul when the bomb exploded. The American civilians worked for the international military force in Afghanistan.

"A lot of dead bodies and wounded victims were there after the explosion happened in the area," said Mohammad Hussain, who was wounded in the attack. "There were a lot of casualties."

A spokeswoman for the U.S.-led coalition in Afghanistan told one of the Americans was killed instantly and the other two died of their wounds at the hospital.

The blast also wounded 66 people, Wahidullah Mayar, a Health Ministry spokesman said.

The causalities reportedly included children, according to witnesses.

Najib Danish, a deputy Interior Ministry spokesman, said the attack destroyed a number of vehicles.

The Taliban quickly denied it was behind in the attack in Kabul's Macrorayan neighborhood, though the militants increasingly have been targeting Kabul in recent weeks and often don't claim attacks that maim large numbers of civilians.

The suicide attack took place at 4:20 p.m. just as schools were letting out for the day outside the civilian Shinozada hospital, just a few miles away from the heavily fortified American embassy, The Washington Post reported.

The explosion could be heard in far-away neighborhoods and prompted the embassy to blare its emergency sirens, the paper said. 

The bombing is the latest in a series of recent deadly attacks following the announcement of a new Taliban leader to replace Mullah Mohammad Omar, whose death was announced by Afghan officials.

On Aug. 7, a Taliban attack on a NATO military base near Kabul's international airport killed an American soldier and eight Afghan contractors. A Taliban-claimed suicide car bombing targeting a NATO convoy on June 30 on the main highway to the Kabul airport wounded two U.S. soldiers and at least 24 others while killing two Afghan civilians.

On May 17, a British security contractor was killed when the armored car he was traveling in for Eupol was hit by a suicide car bomber. Two Afghan women nearby also were killed.

The Associated Press contributed to this report.