The Colombian army has said Marxist guerrillas have bombed the country's main crude export pipeline - the third such attack this year.

The military said that the explosion had caused a large forest fire.

The pipeline is owned by a consortium of British, Canadian, French and Colombian investors.

Largest pipeline

A Colombian army spokesman said the National Liberation Army (ELN) was responsible for the bombing.

According to the army, the explosion happened in the city of Campo Hermoso, in the eastern part of the Colombian Andes, 120 km north-west of the capital, Bogota.

The 800km pipeline runs from the oilfields of Cusiana y Cupiagua to a port by the Atlantic Ocean.

It is Colombia's largest pipeline, providing 405,000 barrels per day.

Foreign involvement

ELN's rebels oppose what they see as an excessive involvement of foreign multinationals in Colombia's oil industry.

Prior to this year's attacks, they had bombed the same pipeline twice since it opened in 1997.

The worst of all attacks was in October 1998. At least 73 people were killed when a fireball of crude oil from the ruptured pipeline engulfed a village.