Alvaro Uribe, the Colombian president, has still not said if he wants to run again for what would be an unprecedented third term in office.
The incident shows that the Farc still has the capacity to launch major attacks despite Uribe's US-backed military campaign aimed at destroying the group.
The Farc is at its weakest in decades after Uribe's campaign to force the rebels back into jungles and mountains. But the guerrilla group remains a threat in rural areas, aided by funds from their involvement in cocaine trafficking and extortion.
In December, the Farc kidnapped and killed a governor of Caqueta statein a rare urban assault. Luis Francisco Cuellar was abducted from his home in Florence by armed men dressed in military uniforms and later found with his throat slit.
The Farc, Colombia's biggest armed group, has been affected by the loss of several senior commanders and a flood of desertions as its fighters come under increasing military pressure.
Colombia's troops are benefiting from better mobility with helicopters, improved training and intelligence, forcing the Farc to resort to ambushes and improvised landmines to attack troops.
The Farc, which is thought to number 6,000 to 10,000 fighters, has been at war with the Colombian government for around 45 years.