Colombian authorities will unearth what is said to be the largest urban mass grave in the world, as part of efforts to find the bodies of scores of people who have gone missing as a result of the country's 51-year-long conflict.

In a slum area of Colombia's second city of Medellin, a team of 30 government officials over the next five months will search for the corpses of civilians killed by all armed groups in Colombia's conflict - right-wing paramilitaries, leftist guerrillas, and state security forces.

"For the size of the area, for the number of people who could be buried in this zone, we believe that there's no other such experience in Colombia or the world, and so it has been said that this is the largest urban mass grave in the world," said Jorge Mejia, an advisor for the mayor's office in Medellin.

The eyewitness statements of several former paramilitary warlords, who demobilized and are now in prison, say that the corpses of around 300 people were dumped in a landfill in the upper reaches of a mountain slum called Comuna 13 in western Medellin from 2002 onwards.

Government officials, though, estimate 90 bodies are hidden in a 24,000 meter-squared landfill, some of whom may have been victims of state security forces.

"All these actors, urban guerrillas, paramilitaries, and some sectors from government institutions, it is possible that they are responsible for what happened in Comuna 13," Mejia said.

The Colombian civil war pitted right-wing paramilitaries, leftist rebels and government troops in a three-way conflict that left hundreds of thousands dead and over a million people displaced.

The government negotiated a disarmament with the paramilitaries and is participating in peace talks in Cuba with the larger of the two leftist rebel groups, the Revolutionary Armed Forces of Colombia (Farc). It has yet to initiate peace talks with the smaller of the leftist rebel groups, the National Liberation Army (ELN).