By TOBY MUSE

Troops killed a senior commander of Colombia's largest rebel army in an air-and-ground raid Saturday, as the U.S.-backed military dealt a stunning setback to the nation's leftist insurgency.

Raul Reyes was the official spokesman for the Revolutionary Armed Forces of Colombia and considered a possible successor to the leftist group's No. 1 spot. The United States had offered a $5 million reward for his capture.

Reyes, 59, died in combat and air strikes in neighboring Ecuador, Defense Minister Juan Manuel Santos told a news conference.

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

There was no immediate reaction from the FARC.

Santos said the military tracked Reyes' location through an informant. The air force bombed a camp on Colombia's side of the border where he was thought be, but as ground troops moved in, they came under attack from another camp across the frontier. Reyes was found dead in the base in Ecuador.

Colombian neighbors Ecuador and Venezuela protested the incursion into foreign territory.

Ecuadorean President Rafael Correa said Colombian President Alvaro Uribe had informed him of the raid, but later announced that he was misled after Ecuadorean officials inspected a bombed rebel camp.

"The (Colombian) president either was poorly informed or brazenly lied to the president of Ecuador," said Correa, who called home the ambassador to Colombia for consultation and promised a diplomatic note of protest.

Venezuelan President Hugo Chavez, who is actively negotiating for the release of FARC-held hostages, warned that any such an incursion into Venezuela would lead to war.

"Don't think about doing that of here because it would very serious, it would be cause for war," Chavez said at a televised meeting with his Cabinet.

The Colombian and U.S. governments accuse the FARC of being one of the largest cocaine cartels in South America, a claim rejected by the group which says it only taxes coca growers and buyers.

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

The death of Reyes, whose real name was Luis Edgar Devia Silva, is the latest in a series of setbacks the rebels have suffered at the hands of Uribe's hardline government, which has vowed to use hundreds of millions of dollars in U.S. military aid to defeat the FARC.

It also was one of the most severe blows to the group since its 1964 inception. Colombian media identified Reyes as the FARC's No. 2 commander.

But Lazaro Riveros, a government negotiator who dealt with Reyes during talks that ended in 2002, cautioned that the FARC will replace Reyes immediately and "will keep moving forward in line with their principles and their structures."

In recent years, Colombian forces have captured or killed top regional commanders with noms de guerre "El Negro Acacio", who oversaw much of the group's coca operations, "Martin Sombra" who is alleged to have guarded hostages, including three U.S. contractors, and "Martin Caballero." In January, FARC leader Ricardo Palmera, better known as Simon Trinidad, was sentenced to 60 years in prison in the United States. But none of these men had the status and rank of Reyes.

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

"This could hit morale (in the FARC) because the myth of the invulnerability of the bosses is over," said Colombia's top security analyst, Alfredo Rangel.

It wasn't immediately clear how Reyes' death would affect 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 endanger 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."

Reyes, who was always accompanied in the jungle with a heavy security unit, 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 part of a more intransigent faction of the FARC, favoring a hardline in talks with the government.

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 how he thought peace talks with the government would end, he thought for a second and then said: "The FARC has never and will never abandon the goal of taking power in Colombia."