"Afghanistan once again saw a day of bloodshed of innocent civilian lives," Ghulam Dastgir Azaad, the governor of Nimroz where the incident occurred said.

The bus was on its way from Nimroz's Delaram district, about 700 kms (430 miles) from the capital, Kabul, when the bomb exploded.

Women and children were among the casualties, the interior ministry said in Kabul, blaming "Afghanistan's enemies" for the blast, a term often used by Afghan officials to describe the Taliban and their militant allies.

A spokesman for the NATO-led International Security Assistance Force (ISAF), whose troops are the primary target of Taliban bombs, condemned the attack.

The incident coincided with the publication on July 22 by the whistleblower group WikiLeaks of tens of thousands of classified US documents which cast a new light on civilians caught up in what the website called "the true nature of this war."