At least 39 members of the Revolutionary Armed Forces of Colombia (FARC) armed group are believed to have been killed by Colombian troops in overnight fighting, military officials have said.
The deaths were announced late on Wednesday night, as mediators said that an often-delayed release of hostages held by the guerrilla group would begin within days.
The joint air force and army operation was "without a doubt, one of the hardest strikes of the past five years", said Juan Carlos Pinzon, Colombia's defence chief.
The main operation reportedly killed 39 insurgents in Colombia's Arauca province, the same eastern area bordering Venezuela where FARC members allegedly ambushed an army unit during the past weekend, killing 11 soldiers.
Officials said three other rebels were killed on Tuesday in Arauca during firefights with the army, and four more were captured.
It is understood that ten hostages held by FARC, six police officers and four soldiers, are expected to be released from March 26, mediators have said.
"The countdown has begun and we have started to discuss when the helicopters will be arriving," former senator Piedad Cordoba told Reuters after meeting with representatives of the Red Cross in Bogota.
"We are trying to speed up the operations, and Brazil is helping enormously. So, I believe this [the extraction] will be done within the expected date."
Reaction has been understandably enthusiastic among the families of those held captive.
"After 12 years and seven months, finally we will be able to hug," Olga Lucia Rjas Medina, the sister of one policemen held hostage, told the Associated Press. "He can have a hug in freedom. It's a blessing."
Meanwhile, the country's president, Juan Manuel Santos, has said he would be willing to open "a direct dialogue" with the leftist movement, but only when all hostages were released and the group vowed to cease "terrorist" actions. He also wants the FARC, at war with Colombia's government since 1964, "to stop recruiting children".
Jorge Bedoya, Colombia's deputy defence minister, said the countdown to a deal would only start once the government had a detailed plan in place with the backing of Brazilian troops.
"This is a process that has security protocols that are quite sensitive," he said. "It is necessary to prepare the
helicopters; the Brazilian government must have a detailed logistics plan and, once we have the departure location of
the aircrafts in the Colombian territory, we will begin the countdown for the liberation."
The defence minister said that 51 FARC fighters have been captured or killed in the past 24 hours as part of the new strategy, known as the "Sword of Honour" war plan.