Colombia's FARC guerrilla group on Wednesday accused former president Alvaro Uribe of being behind alleged spying on peace negotiators by an army intelligence unit.

The alleged spying was revealed Tuesday by the weekly Semana, prompting the firing of two military intelligence chiefs.

Two Colombian military intelligence chiefs were sacked Tuesday after the president ordered a probe of claims an army unit spied on his negotiators holding peace talks with FARC rebels.

President Juan Manuel Santos called the alleged eavesdropping an attempt by "obscure forces" to sabotage his efforts to end the half century old insurgency, denouncing it as "totally unacceptable."

Colombian authorities are investigating allegations that an army intelligence unit spied on Bogota's delegation to peace talks with FARC rebels, a government source said Tuesday.

The magazine Semana reported that the army set up the unit in 2012 to illegally intercept the communications of members of the government's negotiating team.

The FARC rebel group called Monday for a pact with the Colombian government on humanitarian conduct, as the two sides pursue an end to their half-century conflict.

A negotiator for the leftist guerrilla group, Pablo Catatumbo, aired the proposal as peace talks resumed in the Cuban capital Havana.

Negotiators for the government of Colombian President Juan Manuel Santos arrived in communist Cuba Sunday to resume peace talks with their country's leftist rebels.

The team, led by former vice president Humberto de la Calle, is scheduled to sit down with representatives of the Revolutionary Armed Forces of Colombia (FARC) for talks starting Monday following an 11-day recess.

A fire at a Colombian prison killed at least 10 inmates and injured more than three dozen, prison officials said on Tuesday.

The blaze broke out late Monday at a prison in the northern city of Barranquilla, authorities said.

Colombian President Juan Manuel Santos has called for a scientifically backed international effort to weigh the consequences of legalizing marijuana, urging a coherent joint policy to tackle the scourge of drugs.

"We must approach this problem internationally, otherwise we'll take it from one place to another," Santos said, speaking Thursday at the annual World Economic Forum in Davos.

Colombia's FARC rebel central command Friday condemned an attack by its own guerrillas that left a civilian dead, highlighting possible divisions within the group as it attempts to negotiate peace.

"Those responsible never intended to cause injury to the non-combatant civilian population," the FARC secretariat said in a statement on its website.

Uruguay's president said he plans to meet in Havana with Colombia's President Juan Manuel Santos and FARC guerrilla leaders to urge them to accelerate peace talks.

President Jose Mujica's offer to mediate comes as Colombia has stepped up military operations against the rebel group, killing 26 rebels in a series of clashes since the weekend, according to the army.

Colombian troops killed five FARC guerrillas Wednesday in the latest in a succession of military operations that have claimed the lives of 26 leftist rebels since the weekend, a military source said.

The flare-up comes despite peace talks in Havana aimed at ending the 50-year-old insurgency and as the FARC denounced the killing of leftist activists.