A suicide bomber and gunmen in Afghanistan attacked a convoy of U.S. troops who returned fire and 16 Afghan civilians were killed in the violence, the U.S. military said. Afghan authorities said 10 civilians were killed and more than 30 wounded, at least eight of them when U.S. troops opened fire after the blast. They said they were not aware of the convoy coming under fire from gunmen after the blast. Hundreds of angry Afghans protested after the violence, blocking the road near the eastern city of Jalalabad where the attack happened and throwing stones at police. The U.S. military said its troops had returned fire in defence after a "complex ambush" but it did not make clear how the 16 civilians were killed and 24 wounded. The incident was being investigated by U.S and Afghan forces, it said. Earlier, a provincial government spokesman said civilians were killed when the troops in the convoy opened fire after the blast. "Today I was at the district building when the blast occurred. It was around 9:15am when I arrived at the site of the attack I saw the attacker car was completely damaged and the attacker body was at the side of the road," said Esa Khan Zwak, Momand Dara district chief. Provincial police spokesman Abdul Ghafour said the U.S. troops were responsible for the civilian casualties. Deputy provincial governor Abdullah Wafa said the deaths of 10 civilians had been registered. The U.S. military laid the blame on the Taliban. "We regret the death of innocent Afghan citizens as a result of the Taliban extremists' cowardly act," U.S. spokesman Lieutenant-Colonel David Accetta said in a statement. One member of the U.S. force was wounded. The killing of civilians is highly sensitive for foreign troops and the Western-backed government of President Hamid Karzai. "This boy was a sheepherder and there was another child with him. Foreign soldiers opened fire on them, on cars and every one that they were seeing," said Mohammad Agha, a relative of one of the victims. Analysts say bombing and accidental shootings by Western forces feeds resentment of the government and its allies and even bolsters support for the Taliban. More than 45,000 foreign troops are in Afghanistan battling a resurgent Taliban who have threatened a spring offensive after the bloodiest year since their ouster by U.S. forces in 2001.