A Venezuelan-led mission to rescue three hostages held by leftist rebels in Colombia's vast jungles was tottering on the edge of collapse Monday, with guerrillas still not revealing where they'll free the hostages.

Colombian officials accused the rebels of failing to keep their promise to release the hostages, while international observers grow increasingly impatient with the delays, some threatening to return home.

Colombian President Alvaro Uribe was traveling to this central Colombian city to reassure observers from France, Switzerland and five Latin American countries that his government was providing all the necessary security guarantees to carry out the stalled operation, which was originally to be completed by Sunday.

His top peace envoy blamed the delays squarely on the Revolutionary Armed Forces of Colombia, or FARC, which has so far failed to tell Venezuelan President Hugo Chavez exactly where in the France-sized eastern jungles it will free the hostages.

"We are enormously worried over the FARC's attitude of repeatedly lying and not fulfilling its promises," peace commissioner Luis Carlos Restrepo told the 150 journalists who have camped at Villavicencio's airport since Thursday.

The FARC announced two weeks ago that it would free the three hostages — former congresswoman Consuelo Gonzalez, Clara Rojas and her 3-year old son fathered by one of her captors, Emmanuel — to Chavez. The guerrilla group has made no public statements since then.

On Friday, Chavez sent the first two of four helicopters to Villavicencio, but there's been no new movement, other than the arrival of U.S. filmmaker Oliver Stone.

To calm the tensions, Chavez dispatched Venezuelan Foreign Minister Nicolas Maduro. Arriving by surprise in sweltering Villavicencio Monday morning, Maduro said the operation could last several more days and pleaded for "patience and nerves of steel."

Colombia's government had vowed to stay mostly out of the operation, allowing Chavez to use his leftist credentials with the FARC to secure a speedy return of the hostages to family members waiting for days in Caracas.

But the arrival of Uribe, and harsh reprimand by Restrepo, seems to have marked an end to their hands-off approach.

"It appears that the (Venezuelan) communication with the FARC is not working," said Restrepo. "We Colombians have spent 40 years enduring this terrorist group and their lies. ... Naturally, it doesn't surprise us the FARC isn't keeping their promises."

On Sunday, former Argentine President Nestor Kirchner was seen angrily loading his luggage into an SUV and had to be coaxed to stay by panicky Venezuelan diplomats, according to a member of the international delegation who requested anonymity so as not to interfere with the operation.

Several other envoys have also complained privately about the delays and are threatening to abandon the city in time to return home before the New Year's holiday.

Uribe last month abruptly ended Chavez' efforts to broker a wider swap of 44 high-profile hostages — including three American defense contractors and former presidential candidate Ingrid Betancourt — for hundreds of jailed rebels.

But family members have urged the firebrand leftist on, saying he is the only intermediary capable of breaking a government-rebel deadlock.

The two sides have not held face-to-face talks since Uribe took power in 2002.

Uribe has instead used some $600 million in annual military and intelligence aid from Washington to push the half-century-old insurgency deeper into the jungle.