The explosion took on a highway in Delaram district of Nimruz province.
"The roadside bomb struck a civilian bus this morning. Twenty civilians were killed and another 27 have been injured," said provincial governor Ghulam Dastgir Azad speaking with AFP.
He said the bomb had been planted because a coalition convoy was supposed to cross the area this morning.
No group has claimed responsibility for the attack.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ