(CNN) -- Colombian military combat planes opened fire on hidden rebel camps over the weekend, killing at least 20 members of the country's largest guerrilla group just days after negotiators finished a round of peace talks.
A top faction leader of the Revolutionary Armed Forces of Colombia guerrillas was among the casualties, Gen. Mario Valencia told reporters Monday.
The leader, known by the alias "Guillermo Pequeno" (Little Guillermo), had been part of the leftist rebel group for 25 years, Valencia said. In July, Colombia's Defense Ministry offered a 1 billion-peso ($550,000) reward for information leading to his capture.
Military officials described the weekend operation as a devastating blow to the rebel group known as the FARC, which has been at war with the South American nation's government since the 1960s.
The airstrikes in Colombia's Narino department came a day after President Juan Manuel Santos gave a November 2013 deadline for ongoing peace talks with the rebels, which are scheduled to resume Friday.
"This process cannot take years, but months. ... But it is important to be patient and not demand immediate results," Santos said Saturday.
Last week, negotiators from both sides concluded their first round of talks and voiced cautious optimism that they would eventually forge a peace deal.
The FARC announced last month that it would temporarily halt its battle against government forces as a goodwill gesture.
The following day, rebels blew up two electricity towers in Colombia, CNN affiliate Caracol TV reported. The guerrillas later said they carried through with the attack because they were unaware of the cease-fire, Caracol said.
Santos told CNN in September that the government would not participate in a cease-fire while the two sides were in negotiations.
"I've told them there will be cease-fire and we will stop any military operation when we reach a final agreement," Santos said. "And if I see that there's no progress, that they are simply trying to buy time, I will stand up and continue business as usual. And that's why there's no cease-fire, no decrease in our military operations, and my government agenda will continue as it was until then."
There have been sporadic attempts at peace between FARC rebels and officials since the 1980s. The last fell apart in 2002. Then-President Andres Pastrana ceded an area the size of Switzerland to the guerrilla group, but he ended negotiations after rebels launched a series of attacks across the country in an apparent bid to strengthen their position.
A new round of peace talks is scheduled to start in Havana, Cuba, on Friday.
CNN's Patrick Oppmann contributed to this report.