Colombia's leftist FARC rebels announced Wednesday they will observe a one-month unilateral ceasefire in response to an international appeal for an urgent de-escalation in the country's decades-old conflict.

The truce will start July 20, said Ivan Marquez, chief rebel negotiator at long-running peace talks in Havana.

He said the Revolutionary Armed Forces of Colombia (FARC) were acting on an appeal issued Tuesday by four countries supporting the peace talks, in order to dial back half a century of war after a recent spike in combat.

The goal of the truce, Marquez said, is to "create favorable conditions in order to advance with the opposing side toward a bilateral and definitive ceasefire."

Colombian President Juan Manuel Santos welcomed the move. But he said the FARC had to do more to advance peace talks that have been under way in the Cuban capital Havana since late 2012.

"What we want in this country is to end this conflict as soon as possible, and how do we end it? By speeding up the negotiations," he said.

Cuba and Norway are acting as so-called "guarantor" countries in the peace talks. Chile and Venezuela are "escort" countries. All four issued the appeal Tuesday for de-escalation.

- Relative calm -

FARC rebels had been observing a unilateral ceasefire since December, and it led to relative calm.

But clashes resumed in mid-April, following an ambush by the rebels that left 11 soldiers dead. Each side blames the other for the escalation. The FARC ended their truce in May.

During that truce, government forces stepped up attacks on rebel camps, the FARC alleges.

Since that truce ended, about 30 rebels have been killed in army operations and recent surveys show the public is increasingly wary about the peace process.

Three soldiers were killed, four wounded and one reported missing in Colombia on Tuesday following several attacks that were believed to have been carried out by the FARC.

The army confirmed Wednesday the missing soldier is being held by the FARC.

Colombia's civil strife dates back to 1964 and has drawn in left-wing guerrillas, right-wing paramilitaries and drug gangs, killing more than 220,000 people and uprooting as many as six million.

Since the start of the peace talks, the rebels have repeatedly called for a two-way truce. The government has refused, saying that during a previous such truce, the guerrillas used the time to regroup and rearm.

One analyst said the rebels are acting with public opinion in mind.

"This is not a case of the FARC pressuring the government for a bilateral ceasefire. Rather, it is the opposite; they feel pressured by public opinion," said Ariel Avila of the Peace and Reconciliation Foundation, which studies the war.

The peace process has come under fire because it is taking so long and because of the recent flare-up in fighting. So the rebel ceasefire announcement is designed to be "a political lifesaver for the peace process," Avila told AFP.

Despite the renewed bloodshed, the Colombian government said for the first time Saturday that it was potentially open to a bilateral ceasefire.

So far, the two sides have agreed on three points of a six-point agenda for the faltering peace process.

The two sides are said to be close to sealing a partial agreement on reparations to victims of the war, and there is talk this could be announced on Monday when the latest round of negotiations ends.