BOGOTA, Colombia -- The UN special advisor on Colombia, James Lemoyne, warned FARC rebels on Sunday that he could not meet their demands to negotiate an exchange of jailed guerrillas for kidnapped civilians.

"Free the hostages and then we'll sit down to talk about what is a possibility: The exchange of combatants, which has been carried out in many conflicts," Lemoyne told reporters.

"The FARC should change their strategy," he said.

It was the first time Lemoyne has publicly ruled out such a scenario since President Alvaro Uribe asked the United Nations last year to explore a possible "humanitarian exchange" with the rebels fighting a four-decade-old guerrilla war.

The revolutionary armed forces of Colombia, known by it's Spanish acronym FARC, killed 10 of its hostages last week, rather than hand them over to arriving Colombian troops.

Lemoyne called the killings a "massacre," and was shown on Colombian television with his arm around the widow of one of the victims, slain Antioquia gov. Guillermo Gaviria.

Former defense minister Gilberto Echeverri was also shot dead. Surviving hostages said FARC commander El Paisa ordered the execution. "They should recognize that what El Paisa has done is a massacre and ask for forgiveness. There is no excuse," Lemoyne said.

The tough talk appeared to reduce the chances of a quick release of hundreds of kidnapped civilians held by the FARC. The guerrillas are holding many of their hostages for ransom money, but are also keeping political hostages in jungle prisons in the hope of exerting pressure for a swap.

The FARC hostages include Colombian congressmen, a former presidential candidate and three U.S. civilian defense contractors who were snatched from a crashed surveillance plane earlier this year. They had been looking for drug crops.

Uribe, whose father was killed by the FARC in a botched kidnapping attempt in the 1980s, was elected on a law-and-order platform with the campaign slogan: "Firm Hand, Big Heart." His demands for a unilateral cease-fire by the FARC as precondition to peace talks have made him a favorite in Washington.

The United States considers the FARC "terrorists" and drug traffickers and has poured about $2 billion into Colombia's war on cocaine. It has also sent a small contingent of U.S. special forces train Colombian troops in counterinsurgency techniques.

According to Lemoyne, the 17,000-member FARC and the smaller, Cuban-inspired National Liberation Army complain Uribe has no interest in dialogue and the rebels brand him a fascist in communiques. The conflict claims thousands of lives a year.

"We are in total disagreement with the FARC and the ELN in the sense that it is impossible to speak with this government. this is not true," he said.