International troops opened fire on a bus carrying Afghan civilians Monday, killing four people, an Afghan official said, setting off anti-American protests in a key southern city where coalition forces hope to rally the public for a coming offensive against the Taliban.

Elsewhere in the city of Kandahar, a pair of suicide bombers attacked an Afghan intelligence services compound, but were killed after security forces opened fire on them, a local official said. Two intelligence agents and a teacher at a nearby school were injured in the attack, said the head of Kandahar's provincial council, Ahmed Wali Karzai. A local hospital director put the number of injured agents at four.

Kandahar, the largest city in southern Afghanistan, was the seat of the Taliban regime ousted in 2001 and insurgents remain active there despite a heavy presence of foreign forces. Securing it is key to the U.S. military and NATO's aim of turning around the more than eight-year war, but anger stirred by civilian deaths threatens to undercut local support.

Monday's shooting on the bus in Kandahar province's Zhari district left four dead and another 18 people wounded, provincial government spokesman Zelmai Ayubi said. He said international forces took 12 of the wounded to a military hospital. NATO said it was investigating the shooting.

A passenger interviewed at Kandahar hospital, Rozi Mohammad, said they had just left the Kandahar terminal when the bus pulled over to allow an American convoy to pass. Shooting broke out as the third or fourth American vehicle went by, he said, with gunfire coming from the direction of the convoy.

"They just suddenly opened fire, I don't know why. We had been stopped and after that I don't know what happened," said Mohammad, his left eye was swollen shut and his beard and clothing matted with blood. Doctors said he had suffered a head injury but did not yet know how serious it was.

Within hours, scores of Afghans had blocked the main highway out of Kandahar city with burning tires, chanting "Death to America," and calling for the downfall of Afghan President Hamid Karzai, himself a Kandahar native.

"The Americans are constantly killing our civilians and the government is not demanding an explanation," said resident Mohammad Razaq. "We demand justice from the Karzai government and the punishment of those soldiers responsible."

NATO and Afghan authorities declined to identify the international forces involved by nationality.

In the attack on the intelligence compound, Afghan forces with automatic weapons engaged the would-be bombers for 20 minutes, wounding one of the attackers who then detonated his explosives belt, said Ahmed Wali Karzai, who is the president's half brother. The second bomber was also killed, although it wasn't clear if he too had blown himself up, Karzai said.

NATO is gearing up for long-anticipated allied operation to push the Taliban out of Kandahar, from which the hardline Islamic movement emerged as a political and military force in the 1990s.

The top NATO commander in Afghanistan, U.S. Gen. Stanley McChrystal, has issued strict orders to his troops to try to reduce civilian casualties. But these still occur regularly, unleashing raw emotions that highlight a growing impatience with coalition forces' inability to secure the nation.

Kandahar spokesman Ayubi said the provincial government strongly condemned the shooting. NATO spokesman Mst. Sgt. Jeff Loftin said the alliance had dispatched a team to the scene to investigate, but didn't say whether its troops were responsible for the civilian deaths.

Loftin said the local command in Kandahar had no further information on what had happened.

With troop levels rising amid heightened violence, at least 2,412 Afghan civilians were killed in fighting last year, an increase of 14 percent from 2008, according to the United Nations. The U.N. attributed 67 percent of those deaths to insurgents who use ambushes, assassinations and roadside bombs to spread terror, undermine development and punish Afghans seen as cooperating with foreign forces and the Karzai government.

NATO earlier this month confirmed that international troops were responsible for the deaths of five people, including three women, killed Feb. 12 in Gardez, south of Kabul. An Afghan government report on the incident claims U.S. special forces had mistaken their targets and later sought to cover up the killings by digging bullets out of bodies, according to investigators who requested anonymity because they were not authorized to speak with the media.