SANTA ROSA DE OSOS, Colombia -- The families of 10 peasants killed by drug-trafficking paramilitaries buried their loved ones Friday, two days after Colombia's worst massacre in years.

A wake and a service at a cathedral were held for the nine men and one woman slain in the municipality of Santa Rosa de Osos, about 170 miles (275 kilometres) northeast of the capital of Bogota.

Officials say gunmen from a group known as "Los Rastrojos" stormed the "Espana" farm Wednesday when workers were finishing up their day picking fruit. After trying to extort the workers, the gunmen opened fire and tossed a grenade.

"We had never seen a massacre like this," said William Alvarez, a 56-year-old coffee farmer who knew four of the dead labourers. "They were decent, hard-working people."

On Friday, President Juan Manuel Santos vowed to go after those responsible for the massacre.

"Our duty is to chase the criminals of Santa Rosa de Osos," Santos said via his Twitter account. "The Defence Minister is already in the area and we have them in our target."

Authorities had initially offered a $27,500 reward for information leading to the whereabouts of the killers. But after a security council meeting held in Santa Rosa de Oso on Friday, Defence Minister Juan Carlos Pinzon announced that had been raised to $82,500. Security in the area would be doubled to up to 250 policemen and soldiers, he added.

"We need citizens to come together and tell us quickly what happened," Pinzon said. "We need them to give us information that will lead to the capture of these criminals or any other criminals who belong to these gangs."

The morning funeral procession was led by Sergio Fajardo, governor of the state of Antioquia, who shed tears as he walked into the cathedral for a service attended by more than 500 people. Many more crowded the main square, where a day before about 150 farm workers gathered to seek shelter fearing a new attack.

It was the first major massacre in Colombia since August 2009, when 12 members of the Awa indigenous tribe were killed in the southwestern state of Narino. At the time, a prosecutor investigating the case said the killings had been committed by Los Rastrojos.

The group is a violent offshoot of the Norte del Valle cartel involved in drug trafficking, extortion and murder as it competes with other criminal bands that grew out of far-right militias known as paramilitaries.

Colombian police say the gang, which is thought to have hundreds of members, operates on the Pacific coast and along the border with Venezuela.

Several leaders of the Rastrojos were captured last week in the area and officials are blaming the group for the latest bloodshed.

Antioquia police Gen. David Guzman said the murder investigation was on the right track, but he declined to provide details.

Local officials say the owner of the farm was apparently being extorted by the paramilitary group.

Officials have arrested about 2,600 extortionists so far this year, according to the police anti-kidnapping unit. About 60 per cent of them are common criminals and the rest belong to guerrilla groups or organized criminal gangs.