The dawn attack in the central province of Meta takes the total number of FARC rebels killed by the armed forces to 65 after an attack last Wednesday killed 33 rebels who were resting in the northern plains region of Arauca.
"This shows that our armed forces continue their offensive and are not going to stop," Santos said during a meeting of security officials in the provincial city of Villavicencio.
The operations form part of a new military strategy to fight the Marxist guerrillas by destroying their key armed and financial units, marking a shift from the previous focus of tracking down and killing their leaders.
Billions of dollars in U.S. military aid have helped Colombia lead a military offensive that has killed off top leaders of the communist group and pushed them further into isolated mountain and jungle regions.
The FARC's fighting force has dropped by close to half to about 8,000 in the past decade and many of the group's key commanders and founding members are dead.
The new strategy focuses on using intelligence to track down specific battle units and choke off their sources of financing, which include drug trafficking, illegal metals mining and extortion.
The group said last month it would abandon its decades-long policy of kidnapping for ransom and free military and police hostages it holds in jungle camps. The liberation is expected to begin at the start of April.
But the FARC, Latin America's longest-running insurgency, remains a formidable force and continues to attack towns and oil installations in efforts to weaken industries such as mining and energy that have helped Colombia's economy grow.
Just over a week ago, the group killed 11 soldiers in Arauca.
