(Adds four soldiers killed in rebel ambush, Ecopetrol statement, paragraphs 3-5)
BOGOTA, June 17 (Reuters) - Colombia's Cano Limon-Covenas oil pipeline, the country's second biggest by volume, has been shut due to damage from bomb attacks by leftist FARC rebels which caused a significant spill that has contaminated two rivers, the army said on Wednesday.
Revolutionary Armed Forces of Colombia rebels, known as FARC, have intensified attacks on infrastructure in the past few weeks after calling off a unilateral ceasefire. They have attacked pipelines and trucks carrying crude oil, as well as damaging transmission lines, cutting power to large towns.
The army confirmed that four troops were also killed on Wednesday after being ambushed by the FARC who set off remote-controlled explosives and fired on them with machine guns while on patrol in the southern province of Caqueta.
State-run Ecopetrol which operates the Cano Limon pipeline through its Cenit subsidiary, said in a statement it is taking measures to contain the spill as local broadcasters showed footage of a black strip flowing inside the clear river waters.
The company said it was helping to truck clean drinking water to local communities affected by this spill and another in the southwest last week when 3,000 and 4,000 barrels of crude spilled into a river there.
The halting of the 780-km (485 mile) Cano Limon pipeline, which has suffered dozens of attacks in recent years that usually take several days to repair, will not immediately affect Colombia's crude exports as there are adequate stocks at ports.
A source at U.S.-based Occidental Petroleum Corp, whose crude is transported through the pipeline, said its production operations have not been affected. The attacks took place near the Venezuelan border in Norte de Santander province.
The pipeline has capacity to transport 210,000 barrels of crude per day from the northeastern province of Arauca to the Caribbean coast. (Reporting by Luis Jaime Acosta; Writing by Peter Murphy; Editing by Peter Galloway and David Gregorio)