Castro, who turns 80 in August, rarely travels outside of Cuba

Mr Castro, who turns 80 in August and has rarely been seen in public in recent months, arrived in Cordoba late on Thursday wearing a military uniform.

Critics say that his infrequent public appearances are a sign of failing health, but Cuban officials deny this.

Mr Castro met Venezuelan leader Hugo Chavez, a new member of Mercosur, shortly after arriving in Argentina.

Cuba is not a member of Mercosur, but reports say Mr Castro will sign a trade agreement with the bloc.

MERCOSUR Argentina Brazil Paraguay Uruguay Venezuela Associate members Ecuador Chile Bolivia Peru Colombia Profile of Mercosur

Venezuela's admission to Mercosur is set to dominate the agenda at the summit.

Mr Chavez has said he hopes to carry his populist social agenda into membership of Mercosur.

His presence in Cordoba, alongside that of Bolivian President Evo Morales and Mr Castro, is expected to liven up the summit.

"Mercosur has to take up the banner of the struggle against social inequalities, against poverty, against misery, against unemployment, the struggle to satisfy the needs of the people," the Associated Press reported him as saying.

Issues

Among the issues are a high-profile dispute between Argentina and Uruguay over proposals to build a paper mill near the border of the two countries which has been heard at the International Court of Justice at The Hague.

All five Mercosur leaders are left-of-centre politicians

Earlier this month, the court backed Uruguay's plans to build the mill near an Argentine river. Argentina has voiced concerns about the potential environmental costs of the venture.

Bolivia wants to talk to Chile about access to the sea while another lively meeting will involve Mr Chavez and the outgoing Peruvian president, Alejandro Toledo, who have been hurling insults at one another.

A planned transcontinental oil and gas pipeline and the price of Bolivian gas are also on the agenda.

Cordoba was once the home of the Argentine revolutionary, and Fidel Castro's comrade-in-arms, Ernesto "Che" Guevara.