BOGOTA, July 19 (Reuters) - Colombia's U.S.-backed president entered into defense pact with the left-of-center government of Brazil on Saturday, marking a step in regional cooperation aimed at fighting cocaine-funded Marxist rebels.

Conservative President Alvaro Uribe announced the deal in Bogota during a visit by Brazilian leader Luiz Inacio Lula da Silva.

The accord stands in stark contrast with Colombia's diplomatic wrangling with the leftist governments of its other neighbors, Ecuador and Venezuela.

Uribe said Colombia will join the agreement only after receiving assurances that Marxist rebels fighting a 44-year-old insurgency in the country will never be allowed to join the pact, although other governments may participate in the future.

"Given this understanding, Colombia has decided to join the agreement," said Uribe, whose popularity has shot up to over 90 percent since the July 2 military rescue of rebel-held hostage Ingrid Betancourt, a French-Colombian politician.

Colombia's relations with it Ecuador and Venezuela have been troubled over the issue of security. Uribe accuses them of not doing enough to help Colombia fight the rebels while Ecuador and Venezuela have portrayed Uribe as a pawn of the United States.

"By inviting Colombia into the agreement, Lula, as a left-of-center leader, is showing that he understands something that Ecuador and Venezuela do not: that's what's best for Colombia is best for the region," said political commentator Ricardo Avila.

The agreement sets the stage for cooperation in military training, intelligence and weapons procurement. It aims to help both sides police the border between Brazil and Colombia, an area that has been known as a haven for drug traffickers. (Reporting by Luis Jaime Acosta, editing by Chris Wilson)

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