Uribe was shocked at the "madness"

The guerrillas of the National Liberation Army (ELN) did not demand a ransom for the teacher, Ana Cecilia Duque.

Instead, they told her father that if he wanted to see her again, he would have to kill a right-wing paramilitary who lived in the same town - Cocorna in north-western Colombia.

He refused - and after a two-day deadline had elapsed, his daughter's body was found with gunshot wounds outside the town.

I want to be by your side and for us never to be separated because you're my reason for living Duque's farewell letter to her daughter

The news has provoked outrage in a country hardened to violence after 39 years of civil conflict.

President Alvaro Uribe called the killing "madness".

Colombia's human rights ombudsman, Eduardo Cifuentes, said it showed how far the guerrillas and their cause had degenerated.

"We demand the National Liberation Army make amends and that it hand the teacher's murderers over to the authorities as well as anyone else behind this act," he said.

Little sympathy

A farewell letter to the teacher's 10-year-old daughter - which was found by the victim's body - was read out on local television:

Colombia has seen almost four decades of violence

"I miss you and I need you more than ever. I want to be by your side and for us never to be separated because you're my reason for living," the letter read.

The BBC's Jeremy McDermott in Bogota says what little sympathy the rebels had in Cocorna - once an ELN stronghold - seems to have evaporated.

The rebels of the ELN and the larger Revolutionary Armed Forces of Colombia (FARC) have been fighting for almost four decades to overthrow the government and install a Marxist-Leninist regime.

But they have become deeply involved in the drugs trade and kidnapping, and have less than 5% public support.