By Hugh Bronstein

BOGOTA, April 16 (Reuters) - More than 15,000 Colombians were forced to flee their homes in the first 100 days of this year, the worst displacement in the last decade of the country's armed conflict, a human rights group said on Monday.

Fighting between security forces and leftist rebels was the chief source of violence causing an exodus of poor farmers and their families, Colombia's top rights group CODHES said.

U.S.-financed spraying of herbicides on crops used to make cocaine also contributed to the displacements.

Drug-running guerrillas and right-wing paramilitaries move their cocaine production operations into new areas to escape the spraying, displacing farming villages along the way.

"Fumigation causes the expansion of coca growth into new areas," CODHES director Jorge Rojas told Reuters.

Almost 9,000 people were forced from their homes in the town of El Charco in the southwest province of Narino.

"The province is a laboratory of war in which all the factors that generate Colombia's armed conflict come together," the CODHES report said.

The four-decade-old war has caused more than 40,000 deaths since 1990, most of them civilians, while more than 3 million people have been displaced, the United Nations says.

Left-wing rebels say they are fighting for land reform and other measures to narrow the wide gap between rich and poor in this Andean country but they have very little popular support.

President Alvaro Uribe is popular for his U.S.-backed security policies, including a peace deal with paramilitary militias under which 31,000 fighters have turned in their guns over the last three years in return for benefits such as reduced prison terms.

The government says thousands of demobilized paramilitaries, guilty of some of the worst massacres and other atrocities of the conflict, have regrouped into new crime gangs battling for control of lucrative drug-smuggling routes like those in Narino, near the border with Ecuador.

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