Colombia's President Juan Manuel Santos has promised decisive retaliation after leftist FARC rebels killed at least 19 soldiers in a single day in the biggest blow to the military since peace talks began in November.

Country's security forces have launched an operation to "capture or kill" the rebel fighters, authorities said on Sunday.

At least 12 people were captured during the initial stages of the operation, which was launched in response to the killing of soldiers in an ambush the government says was carried out by the Revolutionary Armed Forces of Colombia, known by their Spanish initials, FARC.

"The instructions to our armed forces are: don't stop shooting until we have reached the end of this conflict, because that is how we are going to get to that moment faster, by using blunt force," Santos said.

The instructions to our armed forces are: don't stop shooting until we have reached the end of this conflict, because that is how we are going to get to that moment faster, by using blunt force

Juan Manuel Santos, Colombian president

"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.

In a visit to Arauca state on the Venezuelan border, where 15 members of an army battalion that guards oil facilities were killed in the ambush, Santos ordered the military high command to put "the entire machinery" of war into motion against the FARC.

The other four soldiers killed in combat died in the town of Doncello in the southern state of Caqueta, a traditional stronghold of the FARC.

"Our hearts are with the families of the 15 heroes who sacrificed their lives in Arauca for the tranquillity and security of their fellow citizens," the Colombian president said earlier when confirming the deaths.

At least 70 members of FARC had attacked an army unit, he said.

Local television showed images of the site on Sunday with soldiers seen inspecting the area as an army helicopter circled overhead.

Peace talks

The latest attack has thrown ongoing negotiations between FARC and the Colombian government into doubt.

The rebels had sought a ceasefire when peace talks were launched in Havana, 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.

"If you look at Colombian population, they are not very supportive of the continuation of the talks in this environment, but in many ways this was to be expected, because Santos since the beginning of the talks said that no ceasefire was possible with the FARC until a full agreement was reached," reported Al Jazeera's Alessandro Rampietti from Bogota, the Colombian capital.

"The Colombian army has been very active against the rebels in these past months. According to government figures almost 500 rebels have been captured or killed in these last few months.

"And of course we have seen the rebels ambushing the military as many times as they can."

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.

Source: Al Jazeera and agencies