At least 20 people have been killed and 15 others were injured Saturday in a road accident between the southwestern Colombian cities of Pasto and Popayan when a bus overturned on the Pan-American Highway, police said.

The bus was traveling between the port city of Tumaco, in the southwestern corner of Colombia, and Cali, 320 kilometers (200 miles) to the northeast when the incident occurred.

"Unfortunately, we have a toll of 20 people dead," said Capt. Albertland Agudelo of the Narino department traffic police.

"The hypothesis being investigated is that it appeared to suffer a mechanical failure," Agudelo said.

Images on Colombian television showed a bus had flipped over on the road earlier on Saturday.