Ecuador and Venezuela are leading the opposition to the US-Colombia plan
South American leaders at a regional summit have expressed fresh concerns over Colombian plans to grant American troops access to its military bases.
But at the gathering in Ecuador, they rejected a proposal to formally condemn the proposals, which would allow US access up to seven Colombian bases.
Venezuelan President Hugo Chavez warned that "the winds of war were beginning to blow" across the region.
Colombia says it needs US support to tackle drug lords and left-wing rebels.
The US wants to relocate its base for anti-drug operations in Latin America to Colombia, after Ecuadorean President Rafael Correa refused to extend an agreement allowing US access to a military base in Ecuador.
The Brazilian president, Luiz Inacio Lula da Silva - whose government had previously described the plans as a matter for Colombia - called for a meeting between US President Barack Obama and the region's leaders to discuss their concerns directly.
"As president of Brazil, this climate of unease disturbs me," said Mr Silva, reports AP news agency. "I think we should directly discuss our discontent with the American government."
But during Monday's Union of South American Nations (Unasur) summit in the Ecuadorean capital, Quito, Mr Chavez led criticism of the Colombia-US accord.
The Venezuelan leader warned that the July agreement between Bogota and Washington "could generate a war in South America".
BBC South America correspondent Candace Piette says that in a news conference at the end of their meeting, held in an ancient church in Quito, the 12 presidents looked uncomfortable.
Although a number of countries in the region had previously expressed alarm over the plan, the summit failed to back Venezuelan and Bolivian calls for a joint statement condemning the move.
Instead, Unasur members agreed to hold talks - in Argentina later this month - to discuss the controversial Colombian-US proposal.
Colombian President Alvaro Uribe, a staunch American ally, toured the region last week in an effort to persuade leaders that an expanded US presence would not threaten any other nation in South America.
But correspondents say South American leaders would like firm assurances from Washington that the US forces would not operate outside Colombian territory.
Monday's Unasur summit was held amid growing tensions between Colombia and Venezuela. The Colombian president did not attend the meeting in Quito.
Ecuador severed relations with Colombia after Bogota ordered a raid over the border in March 2008 on a left-wing Farc guerrilla camp.
On Sunday Mr Chavez stepped up accusations against Mr Uribe, saying Colombian soldiers had recently been spotted crossing the Orinoco river, which forms part of the border, and entering Venezuelan territory.
He said the alleged incursion was a "provocation" and put Venezuelan troops on a war footing along the border with Colombia.
The foreign ministry in Bogota said the Venezuelan claims were "not true", because it had checked with Colombian military commanders near the border and they had not reported any such incursion.
"The Yankees have started to command Colombian military forces," Mr Chavez also said on Sunday.
Last week, Mr Obama said the Colombia-US plan would merely update an existing accord, Plan Colombia, whereby US military personnel already help the Colombians fight drug trafficking and left-wing rebels.