Police in southern Pakistan say at least 18 people, including children, died after a fire broke out on a bus carrying people affected by the country's severe flooding.

The fire began on Wednesday night as the bus was traveling on a road in Sindh Province. The blaze engulfed the entire vehicle.

The passengers had temporarily relocated to places near Karachi, a major city in the province.

Many are believed to have been relatives heading home after water receded around their houses.

A man who was on the bus told the Associated Press that the bus suddenly caught fire.

Police say investigations so far suggest the fire spread from the bus's air conditioner.

Heavy rain and flooding since June has left more than 1,700 people dead in Pakistan.