The attack by the FARC guerrilla group on a pipeline in southwestern Colombia earlier this week caused a 20-kilometer (12.4-mile) oil spill that is expected to reach the Pacific coast on Wednesday, Ecopetrol CEO Juan Carlos Echeverry said.

The Revolutionary Armed Forces of Colombia, or FARC, attacked the Transandino pipeline on Monday, causing oil to spill into the Caunapi River, a tributary of the Rosario River, which empties into Tumaco Bay in the southwestern province of Nariño.

The situation is of "extreme concern," Echeverry said in a press conference in Bogota.

State-controlled Ecopetrol implemented its emergency plan, establishing eight spill-control points with a total of 2,200 meters (7,213 feet) of containment booms.

The oil, however, "passed through the control points," Echeverry said, adding that the spill was affecting about 4,000 people who live along the river, as well as mangroves.

The spill has the potential to cause damage in Colombia's Pacific region, where thousands of humpback whales arrive in mid-June, the Ecopetrol CEO said.

Environment Minister Gabriel Vallejo said it could take up to 20 years to heal the environment, assuming "a minimum recovery" of the area.

Ecopetrol operates the 306-kilometer (190-mile) Transandino pipeline, which runs to the Pacific port of Tumaco.

The FARC has staged many attacks in recent years on energy infrastructure on the border with Venezuela and in the southwestern provinces of Nariño, where Tumaco is located, Cauca and Valle del Cauca.

On Monday, FARC guerrillas waylaid tanker trucks and dumped some 200,000 gallons (757,000 liters) of crude oil in Putumayo.

The insurgents intercepted 19 trucks in the wee hours and forced the drivers to pour the oil onto the road.

The incident followed attacks last week by the FARC on electric poles in Valle del Cauca and Nariño, and the northeastern region of Norte de Santander.