Venezuela formally became the fifth full member of Mercosur this month, as the bloc grapples with heated disputes among some member countries.

Chavez, a self-described leftist revolutionary flush with crude oil revenue and a close Castro ally, has a combative relationship with the U.S. government that contrasts with other Mercosur leaders whose ties range from cordial to friendly.

Chavez has promoted Mercosur as an alliance to counter U.S.-backed free-trade deals being promoted by Washington in the region. Castro, in a rare appearance abroad, was traveling to Cordoba to sign Cuba's biggest trade agreement with the bloc.

"Here in Cordoba, today and tomorrow, a new Mercosur will be born," Chavez said after arriving in this central Argentine city, some 435 miles (700 km) northwest of Buenos Aires.

"This will be a relaunching of Mercosur," he added.

Analysts say Venezuela's incorporation could increase tensions in a group struggling to find common economic ground. Full Mercosur members also include Argentina, Brazil, Paraguay and Uruguay. Bolivia and Chile are associate members.

"We'll have to see if in fact these countries will speak the same language or if each one will try to play its own game," said Norberto Consani, an international relations expert at Argentina's Universidad Nacional de la Plata.

Argentina and Chile have clashed recently over energy supplies while Bolivia and Brazil have struggled to resolve differences over natural gas prices. Argentina and Uruguay have also been embroiled in a dispute over the construction of two paper mills along a river shared by the two countries.

CLOSER TIES TO CUBA

Castro, wearing his trademark olive military uniform, arrived in Cordoba on Thursday evening.

On Friday, the presidents were expected to sign a trade agreement with Cuba reducing and eliminating tariffs on a host of Mercosur exports to the communist-ruled island nation.

An appearance by Castro could be seen as an attempt to push Mercosur further left. But moderates like Chilean President Michelle Bachelet, Uruguay's Tabare Vazquez and even Brazil's Luiz Inacio Lula da Silva may resist, as could Paraguay's center-right leader Nicanor Duarte Frutos.

"I don't know if Fidel's presence would be viewed so well by other presidents, maybe by Chavez and (Bolivia's leftist) Evo Morales, but I'm not sure about the rest," Consani said.

Chavez, Morales and Castro are expected to lead a rally by university students and political activists after the Mercosur summit ends on Friday.

Argentine officials insisted Mercosur was capable of transcending any ideological differences and bilateral conflicts.

"Integration is not ideological," said Carlos Alvarez, president of the commission of permanent representatives to Mercosur.

Trade among the bloc's members nearly doubled between 2002 and 2005 to $39 billion, but is still just shy of 1998 levels. Alejandro Mayoral, an international trade analyst, said many Mercosur nations compete rather than complement one another, particularly on commodities.