The FARC have been behind a number of bloody attacks

The authorities say Marxist rebels from the Revolutionary Armed Forces of Colombia (FARC) were to blame for the "barbarous" murder on Monday morning.

The rebels deny the charge, saying the woman, a 53-year-old farmer, was killed by the "forces that want to hurt the peace process."

"I, along with the leadership of the FARC, condemn this murder and I confirm that it is not one of our methods and that none of our people operate in this zone," rebel commander Ivan Rios told a local radio station.

Extortion - or death

The incident happened during a dawn raid by armed men on a dairy farm near Chiquinquira, 80 km (50 miles) north of the capital, Bogota.

A necklace of dynamite around the hope of all Colombians

Andres Pastrana

The intruders demanded several thousand dollars in what the rebel guerrillas call a "war tax" and fixed a bomb to the woman's neck after she refused to pay.

The "necklace bomb" exploded after bomb disposal experts who were called to the scene spent nearly nine hours trying to defuse it.

The woman, Elvira Cortes, died instantly. A bomb squad expert later died in hospital. Three soldiers were maimed.

In April, FARC leaders issued what they called "Tax Law 002," warning that they would step up their campaign of kidnappings and extortion against the rich.

President's statement

Cocaine: Marxist rebels control many drug-producing areas in Colombia

It is the first time that the government has called off the current negotiations aimed at ending a civil war in which about 35,000 people have been killed in 10 years.

"The FARC's attitude must change," said President Pastrana as he announced the suspension. "Forty million Colombians who want peace are demanding that change."

Representatives from 21 countries, including the United States and Europe, had been invited to attend the two-day round of talks on illicit drug crops, which was due to start on 29 May.

The talks began 18 months ago and have been plagued by delays.

The FARC pulled out in early 1999 for several months, demanding that the government to crack down on illegal ultra-right paramilitary gangs.