Alvaro Uribe (L) and Luiz Inacio Lula da Silva held two hours of talks
Colombia's plans to allow US troops to use its military bases is a sovereignty issue, Brazilian President Luiz Inacio Lula da Silva has said.
But in talks with Colombian leader Alvaro Uribe, President Lula requested more transparency on the plan, Brazil's Foreign Minister Celso Amorim said.
Peru has expressed support, while Chile and Paraguay said the accord was a matter for Colombia.
Ecuador, Bolivia, Uruguay and Venezuela have expressed their disapproval.
Argentina's President Cristina Fernandez de Kirchner told Mr Uribe the bases were "not helpful".
Venezuelan President Hugo Chavez - who cut ties with Colombia last month - has said he fears the move amounts to preparation for an invasion of his country by US forces.
President Uribe has visited several of his South American neighbours over the past three days in a bid to calm fears over his decision to open seven military bases to US forces.
Before their meeting in Brasilia, President Lula had made it clear that he was not happy about the idea of even one US base on Colombian soil.
Mr Amorim said the plan seemed to go well beyond what was required.
Washington wants to use Colombia as a regional hub for operations to counter drug-trafficking and terrorism.
The US has been forced to look for a new base for such operations after Ecuador refused to renew the lease on its Manta base, which the US military was using.
"We reiterated that the agreement with the United States, which is limited to Colombian territory, is something naturally for Colombia's sovereignty," Mr Amorim said after Thursday's talks.
But during his two-hour meeting with Mr Uribe, Brazilian President Luiz Inacio Lula da Silva said the matter could have been handled more transparently, according to Brazilian media.
Hugo Chavez is a vocal opponent of Colombia's accord with the US
The Brazilian leader is also said to have asked for guarantees that the actions of US troops would be restricted to Colombian territory.
Even US President Barack Obama's National Security Adviser, James Jones, conceded on a visit to Brazil this week that a better job could have been done when it came to preparing the ground for the agreement, our correspondent adds.
During this week's whirlwind tour of South America, Mr Uribe steered clear of Ecuador and Venezuela, both of which have tense relations with Bogota and Washington.
Mr Uribe has accused Ecuador and Venezuela's leftist leaders of links with the rebel Revolutionary Armed Forces of Colombia (Farc), who have been seeking to overthrow the Colombian governments for 45 years.
Colombia's accord with the US is expected to be raised again when Ecuador hosts a regional summit on 10 August. Mr Uribe and his foreign minister do not plan to attend.