An emergency session was called specifically to address the US deal
Left-wing South American leaders have condemned US plans to increase its military presence in Colombia at an emergency summit in Argentina.
Venezuelan leader Hugo Chavez said it was part of a US strategy to dominate the region. Bolivia and Ecuador also said the deal was a threat to peace.
But Colombian President Alvaro Uribe defended the plan, which will give US forces access to seven military bases.
Mr Uribe said it was needed to combat drug-traffickers and insurgents.
"We are not talking about a political game, we are talking about a threat that has spilled blood in Colombian society," he said after holding up pictures of victims of attacks by leftist Farc rebels.
Moderate leaders, including President Luiz Inacio Lula da Silva of Brazil, have expressed reservations about the plan but are trying to ease tensions between Colombia and its neighbours.
President Lula again called for a meeting with US President Barack Obama and presidents of the Union of South American Nations (Unasur) grouping of South American leaders.
An attempt by Bolivian President Evo Morales to get the other presidents to sign a declaration rejecting the US deal with Colombia was itself rejected, the AFP news agency said.
Instead, the final declaration from the Unasur summit in the Argentine mountain resort of Bariloche said the 12 presidents "reaffirm that the presence of foreign military forces must not... menace the sovereignty and integrity of a South American country and in consequence regional peace and stability," AFP added.
The imminent US-Colombia pact has raised tensions on the continent, with Venezuela's Hugo Chavez characterising it as a near declaration of war and questioning the motives behind it.
"This is part of a global strategy of domination by the United States. That's what's this is," Mr Chavez was quoted as saying by Reuters news agency.
US Secretary of State Hillary Clinton earlier sought to reassure Latin American nations about the planned 10-year accord.
"It does provide the United States access to Colombian bases but command and control, administration and security will be Colombia's responsibility," she said earlier this month.
She said Washington has no intention of significantly increasing its troop numbers in Colombia or establishing a US base there.