Juan Manuel Santos also ordered more troops to the eastern Arauca region, where the attack by suspected Farc gunmen took place on Saturday.

Bogota and the Farc are currently holding talks in Cuba to end more than five decades of conflict.

The government has rejected a rebel call for a truce during the talks.

It says a ceasefire would only provide the opportunity for the left-wing rebels to regroup and rearm.

"I instructed our forces not to stop shooting until the conflict is over," said Mr Santos, who is now in the Arauca region for a security summit in the town of Tame.

"These attacks are not the way. They will be confronted forcefully.

"All of Colombia must work for peace precisely so that incidents like those that occurred in the last 24 hours never happen again," he said adding that the peace talks in Cuba should continue as normal.

The incident happened in a rural part of Arauca known as El Mordisco, when more than 70 Farc rebels attacked a group of 26 soldiers guarding an oil pipeline.

The Colombian authorities said 15 government soldiers died in the attack, although earlier report had put number of the killed at 17.

Six rebels were reported to have died and another 12 seized by the army.

In a separate incident in south-west Colombia, clashes killed four soldiers and several Farc fighters.

The peace talks, which started in November, represent the fourth attempt at a negotiated peace deal since the beginning of the conflict in the early 1960s.

The last attempt to achieve peace collapsed in 2002, when it became clear that the rebels had become stronger during more three years of negotiations under a ceasefire.

Meanwhile, the Farc said it had captured a former US soldier and was willing to release him to prove its commitment to the peace process.

In a statement, the rebels named the man as Kevin Scott Sutay and said he had been taken on 20 June in El Retorno in the southern Guaviare Department.

The US has denied he was on any form of military operation, saying he was in the area as a tourist and had "nothing to do with Colombia's internal conflict".

The Farc are thought to have some 8,000 fighters, down from about 16,000 in 2001.

The government estimates that 600,000 people have been killed since the conflict began with some three million more internally displaced by the fighting.

Last week, the Farc's chief peace negotiator said the conflict was nearing its end.

Ivan Marquez, who is taking part in the Cuba talks, called on left-wing parties and unions to join the effort to achieve peace.

The government wants to sign a peace accord by November. But Mr Marquez warned against rushing into a settlement.