The civil war kills more than 3,000 people every year

The two sides agreed to open negotiations on a permanent ceasefire in January.

Contact between the ELN and the Colombian Government broke down in August, but resumed in November.

The country's biggest left-wing rebel group, the Revolutionary Armed Forces of Colombia (FARC), has not declared any truce for Christmas so far and correspondents say it is unlikely to do so.

Three years of peace talks between the government and the FARC have gone nowhere.

The ELN's announcement, however, is seen as an unexpected boost for Colombia's stalled peace process.

President Andres Pastrana, who was elected on the promise that he would negotiate a solution to the country's 37-year civil conflict, is desperate for progress.

The ELN - hard hit by the armed forces and right-wing paramilitaries - wants to hammer out a deal while it still has some military leverage left.

But the BBC's Jeremy McDermott in Colombia says nobody expects a lasting settlement to be signed before President Pastrana hands over power to his successor next August.

The best he can hope for is that the peace talks with the ELN and the FARC both advance to such a stage that there can be no turning back.

Car bomb

In the meantime, the violence looks set to continue, .

In the latest incident, a powerful car bomb exploded in the town of San Martin, killing at least one person and injuring a number of others.

Police in San Martin, about 150 kilometres (95 miles) east of the capital, Bogota, said the blast damaged several homes and offices.

Mayor Luis Eduardo Diaz told local media that houses as far as eight blocks away were damaged.

The authorities say they do not know who carried out the attack.

San Martin lies just outside a large territory controlled by the FARC.

A right-wing paramilitary group is also active in the area.