Colombia's President Juan Manuel Santos on Monday urged the country's second-largest rebel force, the National Liberation Army, to join the FARC's one-sided ceasefire and parallel peace talks.

"We appeal to the ELN (its Spanish-language initials) to join the unilateral ceasefire as the FARC have done," Santos said.

Colombia's FARC guerrillas on Thursday announced that they would release a soldier they captured last week "in the coming days," calling the move a humanitarian gesture.

The decision came in the wake of a unilateral ceasefire the leftist rebels began Saturday during peace negotiations to end Latin America's longest-running insurgency after more than 50 years.

Colombia's FARC guerrillas killed five soldiers Friday in an attack that came on the eve of a unilateral ceasefire hailed as a key step in peace negotiations, the army said.

FARC fighters ambushed a patrol in the rural area of Santander de Quilichao in western Colombia, an attack that also left five wounded, said a military source speaking on condition of anonymity.

The White House fence should be immediately raised by four or five feet to prevent people from breaking into the grounds, according to an independent security review made public Thursday.

A public summary of the panel's classified report also warned that the Secret Service -- the U.S. agency that guards the president -- is "starved for leadership" and needs root and branch reform.

Colombia's president on Thursday rebuffed a ceasefire by the FARC guerrillas, calling it a gift with "thorns," as an attack by rival rebel group ELN dashed hopes of a respite from the conflict.

President Juan Manuel Santos welcomed Wednesday's declaration of an indefinite unilateral ceasefire by the Revolutionary Armed Forces of Colombia (FARC), but stuck to his refusal to declare a bilateral truce.

Colombia's FARC guerrillas declared an indefinite, unilateral ceasefire on Wednesday in the 50-year conflict, saying they would only use weapons if they came under attack by the army.

The announcement -- the third year running the leftist rebels have declared a ceasefire over the holiday season -- comes soon after peace talks with the Colombian government resumed, following a crisis triggered by the capture of an army general on November 16.

U.S. Secretary of State John Kerry urged the Colombian government and FARC guerrillas Friday to make faster progress in peace talks and said the United States would try to help.

Colombian President Juan Manuel Santos called Tuesday for serious peace talks when negotiations between his government and Marxist FARC guerrillas resume this week to ensure a "happy ending."

Santos' office said government representatives and members of the Revolutionary Armed Forces of Colombia (FARC) would resume talks in Havana on Wednesday.

Five children were among 10 people killed Wednesday when their small plane crashed in central Colombia after developing technical problems, officials said.

The plane's pilot had asked for an emergency landing at Mariquita airport but the aircraft never got that far, said Colonel Carlos Roche, deputy director of Colombia's aviation authority.

Venezuelan prosecutors charged a prominent opposition leader with conspiracy Wednesday in relation to an alleged plot to assassinate leftist President Nicolas Maduro.

Maria Corina Machado, a vocal supporter of anti-government protests that rocked the country earlier this year, vehemently rejected the charge as she left the attorney general's office after questioning.