A prominent Colombian land-rights activist has been killed on her farm, in one of 28 areas where the FARC rebels are to hand over their weapons under a historic peace deal, officials said Thursday.

Cecilia Coicue, a leftist peasant leader, was found stabbed to death Wednesday on her farm, El Vergel, in western Colombia, the national rights ombudsman said in a statement.

The killing raises concerns about the peace process just as Colombians prepare to vote in an October 2 referendum on a deal to end 52 years of war between the government and the Revolutionary Armed Forces of Colombia (FARC).

If the deal is adopted, the Marxist guerrillas have pledged to leave their mountain and jungle hideouts and turn in their arms at the 28 demobilization areas -- including Coicue's 70-hectare (175-acre) farm and the hamlet of Corinto, where it is located.

Her death triggered fears of a return to the politically motivated rural violence the peace process aims to end for good.

"It's a very worrying situation," army general Alberto Jose Mejia told Colombia's Blu Radio.

Coicue, a 62-year-old mother of four, was a member of a leftist movement called Patriotic March and of various farmworkers' organizations.

Six other people have died in "targeted killings" in recent days in the Cauca region, said the rights ombudsman's office, which has sent a commission to investigate.

Deep inequality in the countryside and lack of access to land have helped fuel the violence in Colombia.

The conflict, which has drawn in left- and right-wing armed groups and criminal gangs over half a century, has killed an estimated 260,000 people and left 45,000 missing.

The government has yet to open peace talks with a smaller guerrilla group, the National Liberation Army (ELN).