Leaders of South America's Mercosur trade bloc began a two-day summit aimed at achieving better economic integration and calming concerns about Venezuela's recent entry in the group.

Presidents of members Argentina, Brazil, Paraguay and Venezuela were on hand for the meeting, with Uruguay's leader expected later Thursday. They were joined by associate Mercosur members Bolivia and Chile, and observers Colombia, Ecuador, Surinam and Guyana.

Bolivia since December has been knocking at Mercosur's door and the trade bloc on Thursday created a working group to draft a detailed process of adhesion for the landlocked country within 180 days.

The care given to Bolivia's desired membership was in stark contrast with Venezuela's controversial access to Mercosur, during a special July 4 meeting of the trade bloc, that ill-defined Caracas' economic integration to the bloc.

Venezuela's quick membership also left Mercosur open to criticism it was giving leftist Venezuelan President Hugo Chavez a major stage for his radical political views and often virulent anti-US rhetoric.

Venezuela's oil-rich economy, however, was more conspicuous at Thursday's meeting, with Brazilian Foreign Minister Celso Amorin saying Caracas would strengthen the trade bloc's energy integration and Caribbean relations.

Venezuela is the world's eighth-largest oil producer and fifth-largest oil exporter.

Chavez, in turn, sought to allay concerns, telling reporters he was not seeking to "poison" the trade bloc, but that he would like to see it retooled.

"I have not come to inject ideology into Mercosur; what has fatally polluted us is neoliberalism," said Chavez, whose government is seeking a "21st century socialism."

Chavez argued the trade bloc needed structural reforms and a resetting of goals.

He supports the Bolivarian Alternative for the Americas, a regional group with Cuba and Bolivia, intended as a counterweight to the planned Free Trade Area of the Americas backed by the United States.

Possible trade agreements between Washington and Mercosur members Paraguay and Uruguay were also a matter of concern for Brazilian President Luiz Inacio Lula da Silva, who said they offered "short-term" benefits but long-term "frustrations and many disappointments."

Smaller countries regularly argue they do not get enough out of the customs union and many are seeking bilateral trade deals outside the region.

Lula said that finding a way for members large and small to benefit was critical to Mercosur's success.

"Without integration, Latin America has no way forward," Lula said, adding: "If we do not understand the imbalances among us we will walk away frustrated from every meeting."

Brazil is seeking unilateral trade agreements with Paraguay and Uruguay, which Argentina strongly opposes fearing they could lead to a flood of cheap foreign imports in the region.

On the sidelines of the meeting, Chavez and Lula took an important step toward regional energy integration by signing an agreement to build the first section of an ambitious 20-billion dollar, 8,000-kilometer-long (5,000-mile-) gas pipeline linking Venezuela, Brazil, Paraguay, Uruguay and Argentina.

The initial section of the Great Gas Pipeline of the South will link the cities of Guira, in northeastern Venezuela, and Recife, in northeastern Brazil. Work was expected to begin in 2008 and the pipeline should be completed by 2012.

Argentina, Brazil, Paraguay and Uruguay formed Mercosur in 1991 with the aim of creating a South American common market. Chile and Bolivia became associate members in 1996.

With Venezuela, the bloc now has a total population of more than 250 million people, a gross regional product of over one trillion dollars and regional trade surpassing 300 billion dollars.