President Santos has said he is determined to defeat the rebels Colombian left-wing rebels have killed at least six police officers in an attack near the border with Ecuador.
The Farc rebels used gas canisters stuffed with explosives to bombard a police station in the Putumayo region.
Fighting continued in the surrounding jungle. At least two guerrillas are reported dead.
It is the latest in a series of rebel attacks that have killed more than 30 security personnel since President Juan Manuel Santos took office a month ago.
Analysts say the Farc and the smaller ELN rebel group may be seeking to show the new president that they are still a force to be reckoned with.
But President Santos said the latest attack would not weaken his resolve.
“If they think that with an attack like this they are going to weaken us, they are completely wrong,” he said.
“We will respond with more force and more determination. We are not going to rest a single second until we have peace in this country.”
The attack happened outside the town of San Miguel, close to a bridge linking Colombia with Ecuador.
A bi-national security commission is to investigate whether it was launched from inside Ecuador.
The police said they had foiled a rebel plan to occupy the town after intercepting radio communications.
Troops and air force planes were sent to pursue the rebels in the jungle.
The Putumayo region is a traditional stronghold of the Farc, as well as a major cocaine-producing area.
Both the Farc and the ELN were severely weakened by the tough security policies of President Santos’s predecessor, Alvaro Uribe.
Both groups have indicated that they are prepared to begin peace talks.
But Mr Santos has said that he is only prepared to talk to the rebels if they release all of the hostages they hold and stop attacks.
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