Experts were battling Wednesday to prevent environmental damage after suspected FARC rebels blew up a stretch of Colombia's second-biggest oil pipeline, authorities said.

Separately, also between midday and late Tuesday, the Marxist guerrillas attacked an army patrol in another part of the country, killing four troops and injuring four others.

"The FARC (Revolutionary Armed Forces of Colombia), using improvised explosive devices, were able to stop the flow of crude" in one area in Norte de Santander department, in Colombia's north, the army said in a statement.

The amount of oil spilled from the Cano Limon-Covenas pipeline has not yet been determined. But the army said experts were trying to keep the oil from sullying local water supplies.

State oil company Ecopetrol said work was under way on environmental damage control.

Meanwhile in Cartagena del Chaira, in Caqueta, in the southwest, rebels launched an attack on an army patrol "using explosives installed by terrorists of the 63rd Front of the FARC's southern flank," the army said.

Fighting in Colombia has intensified in recent weeks since the unravelling of a unilateral ceasefire declared by the FARC, which had been hailed as a sign of progress in the peace talks the rebels have been holding with the government since November 2012.

The Colombian conflict has killed more than 200,000 people and uprooted more than six million since the FARC was founded in 1964.