Venezuela alleged Tuesday that Spain's former socialist prime minister Felipe Gonzalez had joined a plot of ultraconservatives to overthrow President Nicolas Maduro's embattled government.

Leftist Maduro, whose country is in economic freefall after adopting a government-run economy while oil prices slide, said the leftist Spanish statesman had signed up with a group of far-right plotters in Spain, Colombia and the United States.

Three FARC guerrillas and one soldier were killed in a military operation in central Colombia, the army reported Sunday.

The clash took place Saturday in the department of Meta and one guerrilla was also taken captive, the military said in a statement.

Colombia's leftist FARC guerrillas urged President Juan Manuel Santos Thursday to intervene to save their unilateral ceasefire, saying ongoing army attacks put it at risk.

"We want to ask President Santos to do something to save the unilateral, indefinite truce the FARC has declared to the country, to immediately stop these operations against guerrilla forces," said Ivan Marquez, the rebels' chief negotiator at ongoing peace talks with the government.

The White House this week was sent a letter that tested positive for cyanide, U.S. media reported Tuesday, in yet another piece of bad news for the Secret Service.

"On Monday 3/16/15, an envelope was received at the White House Mail Screening Facility. Initial Biological testing was negative; however, on 03/17/15, the chemical testing returned a presumptive positive for cyanide," Secret Service spokesman Brian Leary said in a statement on CNN. "The sample was transported to another facility to confirm the results."

Six suspected FARC fighters were killed in military operations in Colombia this week, the army said, at a fragile time when the leftist guerrilla group is conducting a unilateral ceasefire.

According to an army statement, military operations in the southwestern department of Huila and the central region of Tolima also resulted in the capture of 18 other guerrilla suspects.

Eight people were hurt in a bomb attack against a police convoy in Bogota on Thursday, authorities in Colombia said.

Five officers and three civilians were injured when explosives that had been planted on the side of a road detonated just as the police convoy was passing by.

A strong earthquake that rocked northeastern Colombia damaged some 300 buildings but injured only one person, an official assessment said Wednesday.

Most of the damage from Tuesday's quake occurred near the epicenter in the province of Santander.

Colombia will stop air raids on Marxist FARC rebels for a month, President Juan Manuel Santos announced -- a major stride in a peace process aimed at ending Latin America's longest-running civil war.

Peace talks have been under way since 2012 but the president's gesture was an unprecedented step towards ending the conflict, which has killed more than 200,000 people and uprooted more than five million.

Colombia's second largest guerrilla group, the National Liberation Army, on Tuesday freed four geologists it had been holding hostage since mid-February, the International Committee of the Red Cross said.

The four -- three Colombian men and a woman -- were turned over to a delegation led by the ICRC and a representative of the Catholic Church in the northwestern province of Norte de Santander, the ICRC said in a statement.

Colombian army generals met commanders from leftist guerrilla group FARC for the first time Thursday to begin work on a ceasefire proposal aimed at ending their five-decade conflict.

The meeting marks the first time that military commanders from both sides have sat down together to discuss an end to the guerrilla war.