By SIMON ROMERO

CARACAS, Venezuela — Colombia’s Defense Ministry announced Saturday that security forces had killed a senior commander of the country’s largest guerrilla group in combat along the southern border with Ecuador.

The death of Raúl Reyes, one of the rebels’ highest-ranking commanders, was a severe blow for the Revolutionary Armed Forces of Colombia, or FARC, which has been waging an insurgency against the government for the last four decades.

“This is the most important strike yet delivered against this terrorist group,” Defense Minister Juan Manuel Santos said to reporters at a news conference in Bogotá.

Mr. Reyes, the nom de guerre of Luis Édgar Devia, is the fourth senior member of the FARC killed in the past year. Last September, Colombia’s government celebrated the killing of Tomás Medina Caracas, a commander believed to control a large part of the FARC’s smuggling operations in eastern Colombia.

The United States, which provides Colombia with more than $600 million a year in military aid, was offering a $5 million reward for the capture of Mr. Reyes, 59, part of the FARC’s seven-member secretariat and believed to be a contender to succeed the group’s top commander, 77-year-old Manuel Marulanda.

Intelligence operations led security forces to an area in southern Putamayo Province where members of the FARC unit, the 48th Front, were camped Friday night, the Defense Ministry said in a statement. Shortly after midnight on Saturday, Colombia’s air force attacked the camp in a bombing raid.

FARC forces responded with fire from a camp on the Ecuadorian side of the border, the ministry said, killing one Colombian soldier. Colombia’s air attack killed 17 guerrillas, including Mr. Reyes and Guillermo Enrique Torres, a chief theoretician with the FARC, the military said.

Colombia’s government said the bodies of Mr. Reyes and Mr. Torres were recovered in Ecuadorian territory and brought to Colombia on Saturday. President Álvaro Uribe of Colombia informed his counterpart in Ecuador, President Rafael Correa, of the operation, Colombia’s Defense Ministry said.

The military said it conducted its air strike from the Colombian side of the border.

Mr. Reyes often served as a de facto spokesman for the FARC, receiving emissaries and journalists in the group’s jungle camps. He was among the most powerful members of the secretariat, defending the group’s use of abductions and extortion to finance its activities.

The FARC, which also finances itself through cocaine trafficking, is estimated to have as few as 6,000 to 8,000 combatants, down from 16,000 at the time of intense fighting in Colombia in 2001. Still, the FARC remains a major presence in some rural parts of the country.

The focus now turns to Mr. Marulanda, whose real name is Pedro Antonio Marín, who has been reported to be ill. A contender to succeed Mr. Marulanda is thought to be Jorge Briceño Suárez, a hardline commander known as “Mono Jojoy” in the eastern wing of the FARC.

The FARC is believed to hold some 700 captives, including more than 40 political hostages such as Ingrid Betancourt, a former presidential candidate. President Hugo Chávez of Venezuela has helped negotiate the release of six of the FARC’s captives in recent weeks.

Jenny Carolina González contributed reporting from Bogotá, Colombia.