Seven soldiers with Nato's International Security Assistance Force (Isaf) were also wounded in the attack, some of them seriously, Isaf said.

There was no immediate claim of responsibility but similar attacks have been perpetrated by the extremist Taliban movement, which is waging an insurgency backed by al-Qaeda-linked operatives.

The bomber was on foot and blew himself up as a convoy of Isaf troops passed through a bazaar in the town of Dehrawood in the southern province of Uruzgan, provincial police chief General Mohammad Qasem told AFP.

"There was a suicide bombing. Seventeen people -- all of them civilians -- have been killed and around 30 others were injured," he told AFP from the provincial capital Tirin Kot, citing figures provided by police on the ground.

Some of the dead were schoolchildren, he said.

"It was a huge explosion," said a grocer who identified himself only as Subhanullah.

" A large number of people were killed and injured. They were laying on the road," he told AFP by telephone from his shop, which he said was less than 100 metres (yards) from the blast site.

The force of the explosion shattered the windows of his store, and others lining the street, blowing items off the shelves, he said.

Dehrawood is a small town about 400km southwest of Kabul. Subhanullah said the targeted convoy drove off without stopping but returned later with Afghan security forces to investigate.

Isaf confirmed the blast was "likely a suicide attack."

"There are Isaf and civilian casualties," spokesman Major John Thomas said in Kabul, citing a lower death toll as local authorities and adding that the Isaf soldiers were badly wounded.

His information was that six civilians were killed and 13 injured as were seven Isaf soldiers. "All the injured are receiving medical care at Isaf facilities," he told AFP.

The Nato-led force does not give the nationalities of its casualties and would not confirm that the blast was in Uruzgan, where most Isaf troops are Australian or Dutch.

It said the attack was a suicide car bombing.

"This is an indiscriminate use of a Taliban extremist bomb which has killed and injured both civilians and soldiers," said another Isaf spokesman, Lieutenant Colonel Mike Smith.

Tuesday's blast was the deadliest suicide attack since one in Kabul on June 17 that killed 35 people, most of them police trainers.

The Kabul bombing was the worst insurgent attack since the Taliban were driven from government in late 2001 by a US-led alliance.

Civilians are increasingly caught up in the insurgency. About 600 have been killed in the violence this year, according to figures used by the United Nations, around half by Afghan and foreign troops and the rest by militants.

There have been more than 70 suicide attacks in Afghanistan this year, after about 140 in 2006. Most are aimed at the security forces but kill more ordinary citizens.

The violence has intensified this summer with major battles across the country. About 3,000 people have been killed in 2007, most of them rebels, according to statistics compiled by AFP.