Rios was a member of the Farc's seven-man secretariat

Pedro Pablo Montoya, known as Rojas, killed Ivan Rios, a member of the leftist rebel group's ruling body, the Secretariat, then turned himself in.

The Colombian government has made the paying of rewards to informants central to its fight against Farc.

The death came amid a regional row over the killing of another Farc rebel.

Montoya, described by the BBC's Jeremy McDermott in Colombia as a guerrilla with a long and bloody history in the Farc rebel army, shot his boss Rios with a single bullet to the head, and then murdered Rios's girlfriend.

Rojas proceeded to cut off Rios's right hand to take to the security forces to prove he had killed the rebel leader, a member of Farc's seven-man Secretariat.

Controversy

Colombian President Alvaro Uribe said on Monday that they were awaiting the results of an investigation into the killing to determine whether Rojas was entitled to a reward.

"We must wait to see what the judicial authorities say...for them to determine what state the Farc group led by Ivan Rios was in, what was the degree of pressure or threats upon its members...to weigh risk against action taken by the person who is claiming the reward," said Mr Uribe.

If Rojas is paid the $2.5m, it would be part of the government reward scheme for information leading to the capture or killing of wanted rebel leaders or drugs traffickers.

Our correspondent says there is debate in Colombia as to why a killer like Rojas, who would normally be charged with murder, should be rewarded.

The government has answered that it must support the system of rewards that has proved vital to securing intelligence.

Mr Uribe said the scheme was aimed at discovering the whereabouts of the guerrillas but stressed that Colombia, a state of law, was not in the business of encouraging massacres.

Rojas has said his reward would encourage other rebels to kill their commanders and desert.

President Uribe has made the military defeat of the Farc the cornerstone of his government's policy, with the blessing and support of Washington. He has dealt a series of blows to the guerrillas, killing leaders and undermining key rebel units through desertions.

Rios's death came just a week after Farc commander Raul Reyes was killed by Colombian troops in a raid inside Ecuador.

The violation of Ecuador's territory sparked the worst diplomatic crisis in the region for years, which saw Ecuador and Venezuela break off diplomatic ties with Bogota and send extra troops to their borders with Colombia.