Environmentalists contend construction of the network of pipelines would pollute waterways, destroy trees and create roads through the jungle that could draw ranchers and loggers.
At a meeting in Brazil's capital in mid-January, the presidents of Venezuela, Argentina and Brazil promised to prepare in-depth studies for the 10,000km pipeline by mid-year.
The leaders said the pipelines would feed growing regional demand for gas and wean their countries from US economic influence.
Roberto Smeraldi, of the Friends of the Earth-Brazil, said: "I only know what everybody else knows, what has been said publicly."
"A government like Brazil's can't do similar studies for projects covering 500km after 10 years of discussion, and now they are going to manage in-depth studies for a 10,000km project in six months?" he said.
Among other things, the pipeline
will destroy trees, say ecologists
Smeraldi said he believed the pipeline theoretically could be built with minimal impact to the environment, but the cost would be prohibitive.
Hugo Chavez, the president of Venezuela, estimated the pipeline would cost $20 billion to $25 billion, but Smeraldi said strict adherence to Brazil's tough environmental laws would double the cost.
No one at Brazil's Environment Ministry was available to comment on the proposed project.
Fouling the environment
Glenn Switkes, of the International Rivers Network, said if the pipeline were ever built, it would inevitably foul the environment.
"There are a lot of issues involved: direct construction, the question of drainage, all the roads that need to be built," Switkes said.
Roads are particularly devastating to the Amazon rain forest. They allow ranchers, loggers and miners to flood into areas that previously were inaccessible.
"A government like Brazil's can't do similar studies for projects covering 500km after 10 years of discussion, and now they are going to manage in-depth studies for a 10,000-km project in six months?"
Friends of the Earth-Brazil
Environmentalists estimate that each road cut into the rain forest causes destruction of the forest for 50km on each side of the road within a few years.
Analysts also questioned the economic wisdom of the plan, especially after Brazil's government-run oil company announced it would invest $18 billion to develop the country's natural gas fields.
Norman Gall, executive director of the Fernand Braudel Institute of World Economy, said: "Both Brazil and Argentina have gas fields large enough to cover their own domestic demands. I don't see why they would like to undertake this hugely costly project, with money they don't have, not to mention environment costs."
The plan also seems to conflict with other pipeline projects proposed for the region.
"If the government goes ahead with this pipeline (from Venezuela), it will have no money for any other type of investment," Smeraldi said.