The top commander and founder of Colombia's FARC rebel force is dead after more than 40 years fighting against the state from jungle and mountain camps, a government source said on Saturday.

The death of Manuel Marulanda, known as Sureshot, who organized the Marxist-inspired Revolutionary Armed Forces of Colombia guerrilla force in the 1960s, would be the heaviest blow suffered by Latin America's oldest surviving insurgency.

Rumours that Marulanda, whose real name is Pedro Antonio Marin, had died or fallen seriously ill have surfaced before, but they have never been confirmed. He was in his late 70s.

"The information we have is that he is dead. We are not sure of the circumstances, but he is dead," said the top government source who asked not to be identified.

Defence Minister Juan Manuel Santos told Semana magazine a source had told authorities Marulanda died at the end of March. He said there was heavy bombing in the area where officials believed Marulanda was at the time, but added that rebel information said he had died of a heart attack.

Marulanda organized the FARC as a left-wing insurgency fighting for social justice. But after four decades of combat the FARC has been weakened by President Alvaro Uribe's U.S.-backed security campaign.

With little popular support, the FARC has been driven into remote jungle and mountain areas, but remains a potent force in some areas, bolstered by funds earned from cocaine smuggling.

Several top FARC commanders have been killed or captured recently as the rebels struggle against increasing military pressure and growing desertions from their ranks.

Violence from the conflict has eased as Uribe, backed by billions in U.S. military aid, has sent troops to retake areas once under guerrilla control. But using home-made landmines and mortars, the FARC is still battling security forces.

Attempts to negotiate over the release of scores of FARC hostages, including French-Colombian politician Ingrid Betancourt and three Americans, are deadlocked over rebel demands that Uribe demilitarize a rural zone for negotiations.