(Adds comments from Uribe and Colombian defense minister)

By Hugh Bronstein

BOGOTA, Oct 18 (Reuters) - Colombia's conservative president shocked Latin America last week by asking to join a development bank central to Venezuelan President Hugo Chavez's drive for left-wing integration in South America.

But the move by Alvaro Uribe, Washington's top ally in the region, was only the latest in a recent series of friendly gestures toward his anti-American ideological opposite.

As the U.S. Congress reevaluates its aid to Colombia and threatens to reject a free trade deal over human rights concerns, Uribe is reaching out to Chavez, even asking him to mediate a deal to free hostages held by Marxist rebels.

"Uribe is sending the subtle but disquieting message that he can strengthen ties to leftist governments in the region if his friendship with the United States goes bad," said Alfredo Rangel of the Bogota think tank, Security and Democracy. "Washington should listen."

Despite their political differences, Uribe and Chavez are often seen hugging and chatting amiably at public events.

Not all Colombians appreciate the increasingly close ties. Defense Minister Juan Manuel Santo said on Thursday the hostage talks have been a "blessing from God" for Chavez, giving him "a big stage."

Uribe's office quickly issued a statement "reiterating his confidence in the humanitarian efforts of President Chavez."

Uribe, whose father was killed in a botched kidnapping by guerrillas more than 20 years ago, wants to show he is flexible enough to work with Chavez in matters ranging from the "Bank of the South" development bank to energy agreements and talks on a hostage deal with rebels fighting a 40-year insurgency.

Among dozens of high-profile captives are three American defense contractors taken by the guerrillas in 2003. Polls show Colombians want the hostages swapped for insurgents held in government jails, and Chavez has vowed to help broker a deal.

LEFT-RIGHT TANGO

The Venezuelan leader is keen to improve his image in Colombia. If he is successful, it could bolster left-leaning Colombian candidates in upcoming elections.

He has also proposed sending oil through Colombia to the Pacific coast on its way to Asia, and the two countries have a preliminary deal to build the pipeline.

"Chavez wants to reduce oil exports to the United States and increase exports to China. Colombia is key to that plan," said Rafael Nieto, a political commentator and former Colombian deputy justice minister.

"He needs a powerful external enemy such as the United States to justify his revolution," Nieto said. "It is equally important for him to have a good relationship with Colombia."

Colombia has received billions of dollars in U.S. aid aimed mainly at fighting the cocaine trade that has come to feed this Andean country's guerrilla war.

But Democrats on Capitol Hill are tightening conditions on aid and threatening to block the pending free trade deal based on claims that Uribe has not done enough to end violence against trade unionists and other suspected leftists.

The U.S. aid has helped Uribe cut crime and spark economic growth. He won re-election last year after his allies in Colombia's Congress changed the law to allow a second term.

Chavez has also pushed through constitutional changes to extend his time in power and he now wants his allies in Congress to completely scrap presidential term limits.

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