Colombia is on alert after the killing of five human rights activists in recent days, the government said Monday, warning of potential attempts to destabilize a delicate peace process with the FARC rebels.

Land-rights activist Nestor Ivan Martinez was shot dead Sunday in the northern department of Cesar, the fifth rural community leader killed as the government seeks to end a 52-year-old conflict with the Revolutionary Armed Forces of Colombia (FARC).

"We are on alert because in this home stretch of the peace process, there are always groups that want to create instability, create chaos with these kinds of threats and murders," Interior Minister Juan Fernando Cristo said.

Martinez was shot twice in the head while visiting his brother's farm, said an umbrella group of local community organizations in which he was involved.

"They're calling it a robbery, but all his belongings were still there," spokesman Gabriel Henao said.

Pamphlets had circulated in recent days threatening activists who work to help poor Colombians reclaim their land, he added. They were signed by supposedly disbanded right-wing paramilitary groups, he said.

Conflicts over land and class in the countryside have helped stoke the war between the government and the Marxist guerrillas of the FARC.

Their half-century conflict has drawn in various left- and right-wing armed groups as well as criminal gangs over the years, leaving 260,000 people dead and 45,000 missing.

The government and the FARC, the largest rebel group, announced a historic deal to end their conflict last month after nearly four years of talks. It will be put to a referendum on October 2.

But the national rights ombudsman's office has warned of a spate of "targeted killings" of peasants and rural activists ahead of the vote.

Four local community leaders and rights activists have been killed in recent weeks in the western department of Cauca.

They included leftist peasant leader Cecilia Coicue. She was found stabbed to death on her farm Wednesday in one of 28 areas where FARC fighters are due to hand over weapons under a UN-monitored disarmament process.