John Hernandez, alias "Fabian," who was head of finance for Front 17 of the Revolutionary Armed Forces of Colombia (FARC), was killed by government soldiers in a hamlet in the south of the country, Santos said yesterday.
The FARC, who have fought successive Colombian governments since the 1960s and are Latin America's longest-running insurgency, have been in peace talks with the Santos government for more than a year but there has been no ceasefire agreement.
"I said to the lord mayors here that many of my opponents are trying to portray that the government is letting their guard down, the government is handing over the country to communism... There is no such thing."
Santos, who is seeking re-election in the presidential elections on May 25, said that military offensives against the FARC would continue "until the time we get these (peace) agreements."
The conflict is responsible for hundreds of thousands of deaths and displacing some five million people.