Colombia has stepped up its offensive against FARC rebels, killing 21 in only three days, though the government on Tuesday defended peace talks still under way in Havana.

"The military offensive (against the guerrillas) continues at full strength until we reach agreements, as if there were no negotiations at all," President Juan Manuel Santos said on a visit to Madrid.

A flurry of operations against Latin America's longest-running insurgency left 21 rebels dead between Sunday and Tuesday, authorities said.

Troops killed at least seven FARC guerrillas Tuesday in an operation in the central department of Tolima, the military said. The latest fighting came after 14 rebels were reported killed over the weekend in an army air and ground assault on a guerrilla base near the Venezuelan border.

The Colombian government and the FARC have been in peace talks since November 2012, but Santos has vowed to be relentless in pressuring the country's largest leftist guerrilla group.

And the chief government negotiator, Humberto de la Calle, insisted that Bogota -- despite critics' charges -- was not giving away too much to the rebels in a bid to reach a peace deal.

"This could not be further from the truth," De la Calle said in a statement released by a spokesman. He said he rejected suggestions "the FARC is being handed the institutions of the State, and Colombia's democratic tradition being thrown overboard."

A half century of hostilities between the leftist FARC and the government has claimed hundreds of thousands of lives and displaced some 4.5 million Colombians.

Peace talks began in Havana on November 19, 2012 but sporadic attacks blamed on the FARC and military have continued.

The rebels and government delegates are negotiating over a FARC proposal to regulate cultivation and sale of illegal crops such as coca leaf, opium poppies and marijuana.

Illegal drugs are one of six points on the agenda for the negotiations.

Founded in 1964, the Revolutionary Armed Forces of Colombia is Colombia's largest and longest-fighting rebel group, with an estimated 7,000 to 8,000 fighters.