The deaths of two Colombian guerrillas sparked fears this week that the country's fragile peace process could be in peril after voters rejected an accord to end the half-century conflict.

Colombian authorities said the two fighters of the Revolutionary Armed Forces of Colombia (FARC) died Sunday in a clash with the army.

They were the FARC's first fatalities since the government joined it in a bilateral ceasefire in August -- though the rebels cast doubt on whether fighting had taken place.

"This shows how fragile the bilateral ceasefire is," said Ariel Avila, an analyst at the Peace and Reconciliation Foundation.

- Wasted effort? -

The government fell just a few tens of thousands of votes short of passing a historic peace accord in the referendum.

The FARC had gathered its members in special demobilization camps to await the final approval of the accord.

But the "No" vote on October 2 threw their future into uncertainty.

The government and the FARC announced a new deal on November 12, but the level of support it enjoys remains unclear.

Colombian President Juan Manuel Santos himself warned in announcing the new agreement that "the ceasefire is fragile."

"The uncertainty generates fear and increases the risk of this immense effort being thrown away," he said.

- Extortion fears -

Some sources suggested the FARC members were killed while extorting money from civilians, as the FARC have done to raise funds in the past.

Avila warned the ceasefire "could break down... because the conditions for the gathering of the forces are not clear, or because the FARC in their camps are out of money and are trying to raise funds."

Senior FARC commander Carlos Antonio Lozada said the force demanded "a forensic investigation to determine whether there was fighting or not," in a video message released on Thursday.

- UN concern -

United Nations observers are in Colombia preparing to monitor the FARC's demobilization under the hoped-for accord.

The monitoring body, grouping UN, Colombian government and FARC representatives, expressed "concern" over the possible impact of Sunday's deaths.

In a statement released Thursday, the monitor said it was investigating and would make "the necessary recommendations so that this kind of incident does not occur again."

- Power vacuum -

The UN High Commission for Human Rights representative in Colombia, Todd Howland, warned that violence was rising in former conflict zones.

Criminal groups are seeking to fill the "power vacuum" left by the FARC when it regrouped to disarm, he said.

Sunday's violence alone was "not enough to break down the ceasefire," said Frederic Masse, an expert at Bogota's Externado University.

"But it must not be repeated too often," he told AFP.

Masse spoke before the FARC on Friday was forced to deny that dissident members of its force had been involved in another deadly clash reported in the southwest.

- 'Enormous risk' -

The reworked accord tries to address critics' demands to toughen the judicial process against FARC members for crimes in the conflict.

Jorge Restrepo, director of the conflict analysis center CERAC, said those challenging the accord should realize the "enormous risk" of a return to conflict.

"Only disarmament can lead to a complete ceasing of the FARC's criminal activities," Restrepo said.

The multisided 52-year conflict has claimed more than 260,000 lives.

Santos traveled to the United States this week for cancer tests.

The Colombian ambassador to Washington, Juan Carlos Pinzon, later said on radio that Santos was "very well."