Workers reported killed and Afghan troops hurt in suicide car bomb attack in Zabol.
A spokesman for Gulbuddin Hekmatyar, a pro-Taliban leader, said his Hezb-e-Islami group had planned the attack.
Suicide bombings are a favoured method of attack for Taliban fighters opposed to the Afghan government and the presence of foreign troops in the country.
US military officials have said they believe such bomb attacks could rise by 50 per cent this year.
The attack comes a day after US forces said they killed eight suspected Taliban members in fighting in Uruzgan province.
The coalition said two of its troops and three Afghan policemen were wounded during the clash.
The Taliban have been particularly resurgent in southern Afghanistan in recent months.
Barack Obama, the US president, has committed thousands more American troops to the country in an attempt to push back the Taliban.