One of the most senior leaders of the Revolutionary Armed Forces of Colombia (Farc) has been killed, say reports.
Jorge Briceno, also known as Mono Jojoy, died in a military air strike in the Macarena region, known to be a Farc stronghold, local media said.
President Juan Manuel Santos said Jojoy's death was "the hardest blow" in the history of the rebel movement.
The Marxist Farc rebels have been fighting the Colombian authorities since the mid-1960s.
Jojoy was believed to be leader of the Farc's strongest fighting division, the Eastern Bloc, and had eluded Colombian security forces for almost 10 years.
The United States had offered a reward of up to $5m (£3.2m) for information leading to his arrest or conviction.
"A military source has confirmed that it defeated a group of rebels during an operation involving the four branches of the military," Colombia's attorney general Guillermo Mendoza said in a radio interview.
He said soldiers had found the body of Jojoy.
"The symbol of terrorism in Colombia has fallen," Mr Santos said.
"To the rest of the Farc: we are coming after you, we are not going to let down our guard."
A White House spokesman welcomed the news, calling it an "important victory" for Colombia.
Another 20 rebels were also killed in the attack, an official at the defence ministry told the AFP news agency.
The BBC's Jeremy McDermott in Bogota says Jojoy's death will be a major coup for Colombia's new President Juan Manuel Santos.
It also leaves the Eastern Bloc without a leader, which is likely to seriously affect rebel morale, says our correspondent.
The rebels have said they are prepared to find a political solution to the conflict, and have appealed to Mr Santos to enter talks.
But they have stepped up their violent campaign since Mr Santos took office on 7 August, killing more than 40 security personnel in the past month.
Mr Santos has said the rebels must give up their arms and release all the hostages they are holding before talks can take place.
It comes a few days after another guerrilla Farc commander, Sixto Cabana, was shot dead along with 27 other rebels close to the border with Ecuador.
The US State Department said Mr Cabana has been behind the export of hundreds of tons of cocaine around the world and was responsible for hundreds of murders.