Security forces killed the No. 2 commander of Colombia's main guerrilla group in combat Saturday, dealing what the government called the biggest blow to the leftist force since its formation.

Raul Reyes, who became the public face of the Revolutionary Armed Forces of Colombia as the group's spokesman, died in fighting just across the border in neighboring Ecuador, Defense Minister Juan Manuel Santos told a news conference that was carried live on Colombia's main television channels.

"This is the strongest blow dealt to the terrorist group to date," Santos said.

A total of 17 rebels and one soldier died in the operation, which involved Colombia's army, police and air force, the defense minister said. Among the rebel dead was a senior FARC member and songwriter known as "Julian Conrado."

Santos did not say if troops had crossed the border into Ecuador to hunt Reyes, or if they had Ecuadorean permission. A statement from the presidential palace said President Alvaro Uribe monitored the operation throughout the night.

The diminutive 59-year-old Reyes, whose real name was Luis Edgar Devia Silva, was considered a contender to take over the rebel group, known as the FARC.

His death marks the largest setback to the rebels since the hardline Uribe took office in 2002 vowing to defeat them fueled by billions in U.S. aid, and is one of the biggest blows to the group since its 1964 inception.

While top regional commanders with noms de guerre "El Negro Acacio," "Martin Sombra," and "Martin Caballero," have been captured or killed in recent years by Colombian security forces, none had the status and rank of Reyes.

There was no immediate reaction from the FARC to the death of Reyes.

The U.S. State Department had offered a bounty of $5 million for information leading to the arrest of Reyes, as well as the other six members of the FARC's ruling secretariat.

It wasn't immediately clear how Reyes' death would effect efforts to negotiate the release of rebel-held hostages, including French-Colombian president candidate Ingrid Betancourt and three U.S. defense contractors.

In France, those linked to Betancourt expressed fear his death could affect the hostages.

"The international community has its eyes locked on" the FARC, Herve Marro, spokesman for the support committee for Betancourt, said by telephone. "It's in their interest that not the least hair ... of a hostage be touched."

French President Nicolas Sarkozy reiterated his call that Betancourt be freed "without delay." He urged all concerned to emphasize humanitarian considerations and take advantage of the dynamic created by the liberation of four hostages last week.

As the guerrillas' leader, Manuel "Sureshot" Marulanda, grows ever older, Reyes had frequently been mentioned as a potential successor.

"This is heavy blow for the FARC," said Lazaro Riveros, a government negotiator who dealt with Reyes during talks that ended in 2002. "But the FARC ... will keep moving forward in line with their principles and their structures ... he will be replaced immediately."

Reyes, who was always accompanied by a heavy security unit and lived in the jungles, was part of the political wing of the FARC and often took charge of important negotiations, a legacy of his earlier time in the trade union movement. Reyes was considered to be in the more intransigent faction of the FARC, favoring a hard line in talks with the government.

"With Reyes killed in action, this could open the FARC up to taking a more pragmatic line in talks with the government," security analyst Alfredo Rangel told RCN.

Bearded and with glasses, Reyes looked like a small university professor. He seemed dwarfed by the rifle he always carried around, which he would lay across his lap when he sat to talk with a visiting journalist. But meeting him in person, there was no doubting his commitment to armed revolution in Colombia.

When in 2001 an AP reporter asked him what he saw as the end result of the peace talks with the government, he thought for a second and then said: "The FARC has never and will never abandon the goal of taking power in Colombia."

He spoke in a jungle hut, with two armed female guerrillas standing behind. "The political structure of this country is so corrupted that the only solution is to overthrow it and then we can finally have the fair society we've been fighting for the past 40 years."

As well as the death of Reyes, Saturday's strike also killed one of best-liked commanders in the FARC, known as "Julian Conrado."

Conrado composed revolutionary songs that were played at rebel parties and distributed in videos. The quick-to-smile rebel seemed out of place in Latin America's most brutal civil conflict, as he played the guitar, his AK-47 leaning against the wall.

After playing a song once in 2001, he offered one of the thousands of his CDS the FARC had printed to be sold in the zones they controlled.

"Buy one for the revolution," he said.