Several thousand barrels of crude oil have spilled into a river in southwest Colombia after insurgents bombed a pipeline, state-run oil company Ecopetrol said on Wednesday, describing the damage as an "environmental tragedy."

The bomb attack occurred Monday but was not previously disclosed. It was one of spate targeting oil installations this month and will affect several thousand families, Ecopetrol's Chief Executive Officer Juan Carlos Echeverry told reporters. As many as 4,000 barrels of spilled oil have contaminated rivers used for fishing and fresh water supplies.

"It's a social and environmental tragedy," Echeverry said, describing the spill as "senseless." The financial cost to Ecopetrol will be minimal compared with the harm done to the environment and affected communities, he said.

The slick is drifting down the Rosario River, from Pambil in Narino province, and is expected to reach the Pacific Coast by Wednesday evening. The company has deployed booms to recover some of the crude but that will be made harder by rebel presence at along the river's path.

Although Ecopetrol - 88 percent owned by the government - did not name the rebel group, the Revolutionary Armed Forces of Colombia, a Marxist group known as the FARC, operate in the area.

The 300-km Transandino pipeline, attacked six times this year, was not operating at the time of Monday's bombing but it leaked several thousand barrels of residual crude.

The FARC stopped an oil truck convoy in neighboring Putumayo the same day, forcing drivers to dump nearly 5,000 barrels of crude onto the road. Environment Minister Gabriel Vallejo said pollution from that incident could take as long as 20 years to clean up.

Trucks are delivering water to some of the worst affected communities that are unable to draw from rivers.

Echeverry said the attacks had no significant impact on the company's production of around 760,000 barrels per day.

The spill from the Transandino pipeline also risks harming populations of olinguitos, a carnivorous mammal species only discovered in 2013, as the spill will pass close to their known habitat.

The attacks come weeks after the FARC canceled a unilateral ceasefire in retaliation for the government's resumption of aerial attacks on their jungle hide-outs. Peace talks between the two sides are continuing, however.

Echeverry said there have been 20 rebel attacks against Ecopetrol's infrastructure this year, affecting an estimated 84,000 people.