HERAT, Afghanistan (Reuters) - As many as 11 people were killed and more than 30 wounded when a bus in western Afghanistan hit a roadside bomb, officials said on Tuesday.

Mohibullah Mohib, a police spokesman in the western province of Farah, said the incident occurred at about 4.30 a.m. in Farah province on the highway between the city of Herat and Helmand province in southwestern Afghanistan.

He said at least seven people were killed and 27 wounded but a Herat ambulance service spokesman said the number of dead was 11, with 31 wounded.

The victims add to a growing toll from roadside bombs and other explosive devices, which are among the main causes of civilian casualties in Afghanistan.

According to figures from the United Nations, more than 1,000 civilians were killed or wounded by explosive remnants and other roadside bombs and improvised explosive devices in the first half of the year.