* Brazil, Chile join dispute over U.S. troops in Colombia
* Colombia seen growing isolated in Andean region
* Uribe to press his case with South American leaders (Adds Uribe traveling to Brazil, Chile, paragraphs 3, 10)
BOGOTA, Aug 3 (Reuters) - A plan to increase the number of U.S. troops in Colombia is drawing opposition, not just from left-wing populist leaders in the region but from the moderate governments of Brazil and Chile as well.
The mounting criticism threatens to isolate Colombia from its neighbors as it seeks help from the United States to combat drug-running guerrillas and cocaine cartels.
President Alvaro Uribe will tour South America this week to try to ease concerns about the upcoming military pact.
Colombia, Washington's main ally in the region, says the deal is aimed at strengthening anti-drug efforts.
The United States is in talks with Uribe's government about relocating U.S. drug interdiction flight operations to Colombia after being kicked out of neighboring Ecuador. Colombia expects to sign a deal this month after a final round of talks in Washington.
The plan is expected to increase the number of U.S. troops in Colombia above the current total of less than 300 but not above 800, the maximum permitted under an existing military pact, officials said.
Leftist Venezuelan leader Hugo Chavez accuses the United States of setting up a military platform in Colombia from which to "attack" its neighbors.
Chavez allies in Ecuador, Bolivia and Nicaragua were quick to blast the plan as well. But Colombia was shocked late last week when Chile, a model of free-market policies, and regional heavyweight Brazil voiced concern about the deal as well.
"I don't like the idea of an American base in the region," Brazilian President Luiz Inacio Lula da Silva said.
Uribe will meet with Lula, Chile's President Michelle Bachelet and other South American leaders starting on Tuesday. His toughest critics, Venezuela and Ecuador, are not on his itinerary.
Bachelet called the Colombia-U.S. talks "disquieting" and said the proposal should be discussed at the Aug. 10 meeting of the South American Unasur group of nations. But Uribe and his foreign minister do not plan to attend the summit.
The meeting will be held in Ecuador, which has broken off diplomatic relations with Colombia over a 2008 bombing raid targeting Colombian rebels who were camped out on Ecuador's side of the border.
Ecuador and other socialist governments in the region are deepening economic ties with Russia, China and Iran, while denouncing Uribe for his ties to U.S. "imperialists."
"Colombia is increasingly isolated from its neighbors," said Bogota-based security analyst Armando Borrero.
"This has a snowball effect in that it makes the government even more reliant on Washington," Borrero added.
Chavez last week called Venezuela's ambassador back from Bogota over a controversy in which Venezuelan officials are accused of providing Swedish-made anti-tank rockets to the Revolutionary Armed Forces of Colombia, or FARC, rebel group.
Colombian and Swedish authorities asked Venezuela for an explanation after the rockets were found in a FARC arsenal.
Chavez denies helping the guerrillas. His response has been to threaten to nationalize Colombian businesses in Venezuela and to blast the expected U.S.-Colombia pact.
Washington is negotiating to move those operations in Colombia, which has received billions of dollars in U.S. aid to fight the drug trade and the rebels, whose 45-year-old insurgency kills and displaces thousands of people every year.
Colombia is frustrated by the reaction to the talks.
"Where was the hysteria when these operations were being run out of Ecuador?" said a high-level official in Colombia's defense ministry who asked that his name not be used.
"Mexico is having the worst security crisis in its history due to the drug trade and people are saying we should not help them by doing interdiction operations. It's ridiculous," the official said.
The upcoming pact will probably involve an additional "200 plus" Americans in Colombia, including contractors and soldiers, the official said.
U.S. troops in Colombia help plan counter-insurgency missions but are not allowed in combat, a restriction that would not change under a new military accord.
Additional reporting by Raymond Colitt in Caracas, editing by Will Dunham
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