Qayoom Qayoomi, a provincial government spokesman, said the bus was travelling to the provincial capital of Tirin Kot on Friday when it hit a bomb planted in the road.
"Fourteen civilians, mostly elders and children, were killed in the blast and three were wounded," he said.
It was not immediately clear what kind of bomb had caused the explosion or who may have been behind the blast.
Taliban insurgents are behind most bomb attacks, including roadside bombings and suicide attacks, in Afghanistan and they are active in Uruzgan in south-central Afghanistan.
Also on Friday, Nato's security force in Afghanistan confirmed that civilians were killed during a raid on Taliban positions in Kandahar on Tuesday.
A Nato spokesman said: "We confirm at least 12 deaths and we are working with the Afghan ministry of defence to conduct further investigations.
"The possibility that there were others killed in the bombing raid would be part of an investigation that could come to a conclusion in about a week.
Scheffer expressed sadness at
the recent deaths of civilians
Afghan local officials said at least 60 people died in the
Nato raid. Villagers also put the death toll at 60 dead while a member of the Kandahar provincial assembly said that 80 had been killed.
Witnesses said that at least 25 homes had been destroyed in nearly five hours of bombing.
Jaap de Hoop Scheffer, the Nato secretary-general, told George Bush on Friday of his sadness at the recent deaths of civilians in Nato's Afghanistan operation, which the White House said he attributed to Taliban militiamen using civilians as shields.
Captain Andre Salloum, a spokesman for the International Security Assistance Force, said that ISAF had killed 48 Taliban fighters in three skirmishes on Tuesday.
The violence has claimed around 3,000 lives in Kandahar, according to a rough estimate.
In other news, Hamid Karzai, the Afghan president, said on Friday he was prepared to negotiate with Taliban leader Mullah Mohammad Omar for the sake of peace in Afghanistan.
Karzai said: "Those people, if it is Mullah Mohammad Omar or others, if they want to talk and negotiate with us, they are welcome but they should first free themselves from foreign slavery and come to their own land and live in peace.
"For the sake of peace in Afghanistan, we are ready to negotiate with them. We have always negotiated with their supporters and people who control them, and will do again."
The Afghan government and Karzai himself have often accused neighbouring Pakistan of harbouring extremists and insurgents, and even supporting them. Pakistan has always denied the claim.
Karzai said, however, he was not offering amnesty to Mullah Omar or Afghan commander Gulbuddin Hekmatyar, who is also leading a rebelling Islamist faction behind the everyday violence in Afghanistan.