KABUL (Agencies) - American troops raked a large passenger bus with gunfire near the southern city of Kandahar on Monday morning, killing as many as five civilians and wounding 18,sparking furious protests, Afghan authorities and survivors said The attack infuriated Kandahar leaders and could harm public opinion before perhaps the most important offensive of the war, a campaign that is intended to take control of the Kandahar region from the Taliban this summer. Hundreds of demonstrators gathered around a bus station on the western outskirts of Kandahar, shouting anti-American chants and blocking the road for an hour, according to people in the area. The enraged protesters took to the streets of Kandahar and burning tyres and shouting death to America, death to Karzai, death to this government. The American military confirmed the shooting but there were disputes over details, including whether the troops who fired on the bus had first shot flares and warned the driver to stay back. The Nato said it deeply regrets killing civilians, who were shot dead when troops opened fire on a bus in southern Afghanistan. One of the bus passengers and a man who identified himself as the driver said that an American convoy about 70 yards ahead of the bus opened fire as the bus began to pull to the side of the road to allow another military convoy travelling behind to pass. The two convoys and the bus were on the main highway in Sanzari, about 15 miles west of Kandahar city. All of the windows on one side of the bus were shot out. Troops opened fire on the bus just after daybreak as it was taking dozens of passengers to Nimruz Province, said Zalmy Ayoubi, a spokesman for the Kandahar provincial governor. Some of the wounded were in critical condition, and the death toll could rise, local officials said. Ayoubi said five civilians had been killed, including one woman. The Interior Ministry in Kabul issued a statement saying four civilians had been killed and 18 wounded, blaming Nato forces travelling in front of the bus for the shooting. An American military spokeswoman put the toll at four dead - including one woman - and said five people had been wounded. The military spokeswoman confirmed that a convoy travelling west, in front of the bus, had opened fire, but said the second convoy was travelling east-bound towards the bus. She also claimed that immediately before the shooting the troops fired three flares towards the bus to warn the driver he was following too closely, and that one soldier raised his fist in the air as another warning. She also said the driver of the bus was killed. However, the man who identified himself as the driver said the bus did not violate any signal from the troops. I was going to take the bus off the road, said the man, Mohammed Nabi. Then the convoy ahead opened fire from a distance of 60 to 70 yards.