Brazil and Venezuela have signed a series of 26 bilateral agreements to strengthen what they call their strategic alliance.
The Brazilian leader flew into Venezuela on Sunday evening
Brazilian President Luiz Inacio Lula da Silva and his Venezuelan counterpart, Hugo Chavez, signed the agreements in Venezuela's capital Caracas on Monday.
The accords cover areas such as defence, mining and energy.
The two countries' state-owned oil companies have also signed joint ventures for a range of projects.
"Today we have reached the point of no return in the path towards integration," Mr Chavez was reported as saying by Spanish news agency Efe.
"We have the world's largest oil reserves and the continent's largest gas reserves," he said, adding that Venezuela would rather strike deals with other South American countries than with the USA.
As part of the new cooperation accords, Petroleos de Venezuela (PDVSA) and Brazil's Petrobras will jointly build a refinery in north-eastern Brazil and drill for oil in Venezuela.
Mr Lula da Silva stressed the need to build bridges and highways as well as telecommunication networks between neighbouring countries to further integration between South American countries.
Other deals concerned joint agricultural and scientific development as well as commercial trade.
Brazil also offered aid in the wake of floods and mudslides that left thousands homeless in Venezuela.
The two leaders also discussed Venezuela's interest in buying jet fighters from Brazil, which is the world's fourth-largest jet maker.
Trade between the two countries reached $1.6bn in 2004, and is expected to top $3bn this year.