The Farc, suspected of this attack on a bus, is Colombia's oldest rebel group
Two of Colombia's biggest rebel groups have announced they intend to unite to fight the country's security forces.
The Revolutionary Armed Forces of Colombia (Farc) and the National Liberation Army (ELN) said they were "on our way towards working for unity".
Farc and the smaller ELN have deep ideological differences and have fought each other in some regions.
Together they could be a significantly greater danger to the state, says the BBC's Jeremy McDermott.
The surprise announcement was made on a website known for its links with the Farc.
"Our only enemy is North American Imperialism and its oligarchic lackeys," the statement said, a reference to the US which supplies aid and training to the Colombian security forces.
The head of the Colombian armed forces, Gen Freddy Padilla, was dismissive of the news.
"This alliance is impossible," he said. "They dispute territory to control drug-trafficking and have killed one another in the south (of the departments of) Bolivar and Arauca."
Farc has in the past tried to absorb ELN, although the smaller group proved to be stronger than expected, beating back the Farc in several areas.
The Farc is Colombia's oldest and largest left-wing rebel group. It was once thought to have some 16,000 fighters, but reports suggest it now has about 9,000. The group is rurally-based and finances itself through drug trafficking.
The ELN was formed in 1965 by intellectuals inspired by the Cuban revolution and liberation theology. It is regarded as being more ideological than the Farc and has succeeded in recruiting in urban areas. It is thought to have some 1,500 fighters.
It is not clear to what extent the two groups can put aside their differences.
"Now they have something in common, that they have been seriously diminished by Uribe," Mauricio Romero, a political analyst, told Reuters.
But he sees their union as largely symbolic.
The Farc has suffered several defeats at the hands of conservative President Alvaro Uribe's security forces.
Now under new leadership, it is steering a new path, and allying itself with former enemies to try to recover lost ground, our correspondent says.