At least 25 civilians have been killed and another 27 injured in a roadside bombing in southwestern Afghanistan.

The explosion struck a bus travelling on a highway in Nimroz province around 7 am Wednesday (02:30 GMT), officials said.

"The bomb had been planted by the enemies because this morning a [Nato] convoy was supposed to cross the area," Ghulam Dastgir Azad, the provincial governor, said. "Afghanistan once again saw a day of bloodshed of innocent civilian lives."

The bus was passing through the Delaram district, around 700km from the Afghan capital of Kabul, when it was struck.

Roadside bombs, often called improvised explosive devices (IEDs), have proven to be a scourge of the war in Afghanistan.

Classified Nato military records recently releasedto the public by the whistleblower website Wikileaks document at least 7,202 IED explosions between 2004 and 2009, though the majority occurred in the south and east of the country.

According to an analysis of the releaseby the Guardian newspaper, 2,187 civilians were killed and 4,811 wounded by IEDs in the five years covered by the data.