BOGOTA, Colombia - Colombia's cancellation of Venezuelan President Hugo Chavez's mediation with leftist rebels threw into disarray hopes for a prisoner swap that would free three U.S. military contractors and a former presidential candidate.

The decision, announced late Wednesday, took Colombians by surprise but followed a series of increasingly tense exchanges between Chavez and this country's conservative president, Alvaro Uribe.

It also prompted an immediate appeal Thursday from France's president, Nicolas Sarkozy, not to end the go-between role Chavez assumed in August.

Among the 45 hostages with lives in the balance is Ingrid Betancourt, a French-Colombian citizen seized in 2002 while campaigning for Colombia's presidency.

The Venezuelan president defied Uribe by directly contacting Colombia's army chief on Wednesday to discuss the hostages, Uribe's spokesman said in a hastily called late-night appearance.

The official, Cesar Mauricio Velasquez, read a curt statement that also ended the mediation role of a leftist Colombian senator, Piedad Cordoba. The lawmaker had called the army chief, Gen. Mario Montoya, and passed the phone to Chavez.

There was no immediate comment Thursday from Uribe or Chavez. But Venezuela issued a statement expressing frustration, calling Colombia's decision regrettable and expressing a desire to continue offering its "humble services in honor of life and peace."

In France, a spokesman for Sarkozy, whom Chavez visited in Paris earlier the week to discuss the case, appealed to Uribe to reconsider.

"We continue to think that President Chavez is the best chance for freeing Ingrid Betancourt and all the other hostages," the spokesman said.

The American hostages, Keith Stansell, Marc Gonsalves and Tom Howes, were taken by the Revolutionary Armed Forces of Colombia, or FARC, in February 2003 after their small plane crashed in the jungle during a surveillance mission.

"Chavez was the best and only hope," Gonsalves' mother, Jo Rosano, told The Associated Press from her Connecticut home. "The FARC is at least trying. Uribe says he wants an agreement, but he is not to be trusted."

Betancourt and the three Americans are valuable assets for the FARC, which has been fighting the government for more than four decades and is bankrolled chiefly by the cocaine trade.

For their release, Latin America's most potent rebel force was demanding the government free all imprisoned guerrillas, who number in the hundreds.

Uribe had been cool to the FARC's overtures - and unwilling to agree to rebel conditions to create a safe haven for talks on a swap, preferring to try to rescue hostages militarily.

But when Chavez offered to mediate, he agreed.

Those efforts stumbled, however, when Chavez hosted a senior FARC commander in the Venezuelan capital of Caracas on Nov. 8, and the rebels provided no proof of life for hostages, as demanded by the Colombian and U.S. governments.

"I think the FARC has left the entire world wondering if the hostages are dead or alive," said Camilo Gomez, a former Colombian peace negotiator. He also faulted the Colombian government for emphasizing a military rather than a political solution.

Chavez angered Uribe this week by revealing details of a conversation the two had during a summit in Chile that Uribe said was private. Chavez said Uribe told him he was prepared to meet with FARC's septuagenarian leader, Manuel "Sureshot" Marulanda.

In response, Uribe placed a Dec. 31 time limit on Chavez's mediation efforts.

Colombian officials have also expressed continuing concern that the Chavez government is providing the FARC with safe haven.

On Wednesday, Colombia's chief peace negotiator, Luis Carlos Restrepo, said in a TV interview that while in Chile, Uribe reminded Chavez of information he had of alleged FARC encampments across the border in Venezuela.

In Paris the previous day, Chavez told reporters he hoped a prisoner swap might "open the door to a peace accord and that the FARC might form its own political party, its own political movement. Why not?"

Since taking office in 2002, Uribe has tenaciously attempted to defeat the peasant-based rebel army with some $4 billion in U.S. military aid. The Colombian leader has a personal stake: In 1983, FARC guerrillas killed his father in a botched kidnapping.

Relatives of FARC hostages expressed pain and frustration at the latest twist in what for many has seemed an eternal crisis.

"We are dying alive here," said Magdalena Rivas, whose 31-year-old son, police Lt. Elkin Hernandez, has been held by the FARC for more than a decade. "It's a pain so great that I carry in my heart. I'm sad. I'm in pain. I'm seething. I don't know what to do."

Many relatives of FARC hostages are angry at both the government and the rebels for what they consider intransigence on both sides.

They want answers to questions including who was behind the deaths of 11 regional deputies killed by gunfire while in FARC captivity last June. The rebels claim the deputies died during an attack by an unknown military force. Uribe insists there was no rescue mission and that the deputies were killed in cold blood.

- Associated Press writers Toby Muse and Carlos Gonzalez in Bogota, Colombia, and Ian James in Caracas, Venezuela, contributed to this story.