Due to President Juan Manuel Santos' refusal to agree to modify the constitution if a peace pact is struck, Colombia's largest guerrilla army temporarily walked away Friday from peace talks with the government.

It is the first time either side has broken off negotiations that began last year in Havana, other than for planned recesses. But the Revolutionary Armed Forces of Colombia, or FARC, stopped short of pulling out of the peace process entirely.

The move comes after Santos announced Thursday night that he was asking Congress to consider a bill that would let a public referendum on an eventual peace deal coincide with congressional or presidential elections next year. His proposal for the referendum falls short of the constitutional convention sought by the FARC.

The government and rebels have long been at odds on the issue, and Santos' decision prompted the rebels to call a time-out to regroup.

It is perfectly legitimate and valid for (the FARC) to study this proposal...but time passes and the Colombian people's patience has a limit, and we have to keep moving forward in the talks.

- Colombian President Juan Manuel Santos

"The FARC has decided to take a pause from the discussion table to focus exclusively on analyzing the reach of the government proposal, without detriment to the internal consultation it must perform as an organization," the rebels said in a statement read to reporters by Jorge Torres Victoria, a high-ranking leader who goes by the nom de guerre Pablo Catatumbo.

The FARC did not say how long it would stay away from the negotiations.

Humberto de la Calle, Santos' lead negotiator at the Havana talks, said "the government remains at the table. ... We hope (this pause) is brief."

Speaking at a public event in the northwestern department of Choco on Friday, Santos struck an optimistic tone.

"It is perfectly legitimate and valid for (the FARC) to study this proposal," the president said, "but time passes and the Colombian people's patience has a limit, and we have to keep moving forward in the talks."

However he later ordered his negotiating team to return to Bogota to evaluate the guerrillas' move, and said they would resume talks "when we consider it appropriate."

"It is not the FARC that decrees pauses and imposes conditions," Santos said.

On Thursday he presented Congress with a bill to lift the prohibition on referendums being held in the same vote as presidential or congressional elections. Santos is expected to run for a second term in elections next May, and legislative elections are in March.

De la Calle said the aim of the referendum bill is to encourage increased participation by linking it to an election.

"The government remains at the table. ... We hope (this pause) is brief," he said.

Santos ally Simón Gaviria of the Liberal Party said holding the referendum at the same time as a scheduled election would save the nation $93.6 million.

He said he was not concerned by the FARC's suspension of talks.

"It seems to us that, without a doubt, this is a process of negotiation. ... It does not worry us, nor do we think we have to take the FARC into consideration to make these changes (to electoral rules)."

Opposition lawmaker Iván Cepeda of the Alternative Democratic Pole party criticized Santos for apparently making the decision unilaterally without involving the FARC negotiators.

"A decision of this nature pertaining to timing should be fully agreed to by both sides," Cepeda said.

Formed in the 1960s, the FARC is the oldest active guerrilla army in the Western Hemisphere.

Before Friday's pause, talks were focused on the second item on a six-point agenda: the rebels' political integration in a post-conflict Colombia.

Earlier this year a preliminary accord was struck on the first item, agrarian reform, a key issue at the root of the decades-old armed conflict.

"We have made much progress, I must reiterate that point," Santos said. "The agreement we reached on the first point is a transcendental agreement. Nobody, or few, imagined we would reach a deal on that first point."

Based on reporting by The Associated Press.

Follow us on twitter.com/foxnewslatino
Like us at facebook.com/foxnewslatino