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By Patrick Markey

BOGOTA, May 8 (Reuters) - Colombia may review its relations with the United States if the U.S. Congress fails to approve a free trade agreement between the Andean country and its top ally, Vice President Francisco Santos said on Tuesday.

Santos told RCN TV a failure to clear the accord would send the wrong message in the region, where leftist leaders in Venezuela and Ecuador strongly oppose U.S. free market and foreign policy proposals.

He did not specify how Colombia might revise ties with the United States, its top trade partner and source of billions of dollars in aid under the Plan Colombia program to help fight leftist guerrillas and drug traffickers.

"It is a clear message, one that closes doors with the United States and perhaps could lead to a revising of ties on Colombia's part," Santos said according to a transcript of an interview with RCN's late-night Primera Linea program.

"I think it is a subject that has to be evaluated very, very seriously."

The tough remarks came as U.S. Deputy Secretary of State John Negroponte visited Bogota on Tuesday in a show of backing for President Alvaro Uribe, who has been one of Washington's staunchest allies in Latin America.

"My visit is a message of support," Negroponte told reporters at a flower plantation on the outskirts of Bogota, insisting he believed the U.S. trade deal would soon be approved by the Congress.

UNDER FIRE

Uribe is under fire after a scandal erupted linking some of his political allies to illegal paramilitaries accused of atrocities during a dirty war with leftist rebels who are still fighting an insurgency that dates from the 1960s.

While the White House backs Uribe, U.S. Democrats who now control Congress say the scandal and concerns over rights abuses could influence whether they approve a new aid package and a free trade deal for Colombia.

Financed by U.S. assistance, Uribe has helped reduce violence and negotiated the disarming of 31,000 paramilitaries. He says he welcomes investigations into militia ties as a sign Colombia's institutions are working.

But rights groups say paramilitary warlords have kept their criminal networks active.

The Colombian leader recently traveled to Washington to lobby U.S. Democrats to support the trade agreement and the assistance package. But Democratic leaders remained skeptical, especially over the trade deal.

U.S. Sen. Patrick Leahy, a Vermont Democrat who chairs a key aid committee, recently froze $55 million in military aid to Colombia over concerns about militia ties. Former U.S. Vice President Al Gore also snubbed Uribe at a Miami conference citing worries about the scandal.

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