The authorities have blamed the attack on FARC rebels

Fifteen people were injured in the blast in the town of Chita, 240 kilometres (150 miles) north-east of the capital Bogota.

Residents were crowded into the area in preparation for the town's weekly market when the incident occurred, army spokesman Sergeant Luis Hernandez said.

The Colombian authorities blame the country's largest rebel group, the Revolutionary Armed Forces of Colombia (FARC), for the attack.

The rebels have used bombs strapped to donkeys and horses in past attacks.

The FARC and a smaller leftist rebel army have been waging war on the Colombian Government for nearly 40 years and on average 3,500 civilians caught in the crossfire have been killed each year.

The country's President Alvaro Uribe and the Colombian army have recently launched an offensive against the rebels - backed by $3bn of US aid.

In response the 17,000-strong FARC, unable to take the military on head-on, has increasingly resorted to setting off bombs in urban centres.