BOGOTA, Colombia — President Juan Manuel Santos has promised decisive retaliation after Colombia's main rebel band killed 19 soldiers in a single day in the biggest blow to the military since peace talks began in November.

Santos traveled on Sunday to Arauca state on the Venezuelan border, where 15 members of an army battalion that guards oil facilities were killed in an ambush Saturday.

The other four soldiers killed in combat Saturday died in the town of Doncello in the southern state of Caqueta, a traditional stronghold of the Revolutionary Armed Forces of Colombia.

The attacks fell on Colombia's independence day.

The rebels, known by their Spanish initials FARC, had sought a cease-fire when peace talks launched in Havana in November but Santos refused. From 1999 to 2002, the government granted the FARC a Switzerland-sized safe haven in the country's south for peace talks that failed.

In his visit to Arauca, Santos ordered the military high command to put "the entire machinery'" of war into motion against the FARC.

"Just as we have extended our hand and are in negotiations, so do we have a big stick. We have decisive military force and will apply it," he said.

The FARC have been badly battered militarily in recent years and analysts say chances of ending its nearly half-century-old insurgency have never been better.

Both sides say they have reached a preliminary agreement on land reform. But they also say nothing is settled until everything on the six-point agenda is agreed upon.