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 Ecuadorian President Rafael Correa refused to extend an agreement allowing US access to the Manta military base in Ecuador.
Brazilian president Lula 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 Lula adding that "I think we should directly discuss our discontent with the American government."
Venezuela's Chavez warned that the July agreement between Bogota and Washington "could generate a war in South America."
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 Washington 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.
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.
A week earlier the Colombian government said that Swedish manufactured portable rocket launchers, allegedly purchased by Venezuelan Armed Forces, had been seized from the Farc guerrillas.
"The Yankees have started to command Colombian military forces" Chavez also said on Sunday.
Last week, President 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.
Fidel Condemns Colombia
Former Cuban president Fidel Castro called Colombia "disloyal," claiming the pending military deal between the United States and Colombia could be used to attack other Latin American countries.
To say the US military is going to use Colombian army bases to fight terrorism and drug trafficking is an "insult to the intelligence" of Latin America, Fidel Castro wrote in his commentary 'Reflexiones'. "The real goal of the agreement is to control economic resources, to dominate the markets and to fight social changes," Castro added.
"History will not forgive those who commit such disloyalty to their people or those who use 'sovereignty' to defend the presence of Yankee troops."
"What sovereignty are they referring to? That conquered by Bolivar, Sucre, Marti? None of them would have ever accepted such a repugnant argument to justify the granting of military bases to the armed forces of the United States," the 82-year-old former leader said.
According to Castro, the "Yankee military could promote a dirty war….could attack any country but hardly the combative, brave and patriotic Colombia."
"The imperialists underestimate the other countries of Latin America. No country will agree to US military bases," Castro said.
Castro is one of many Latin American politicians who have raised their voices against Colombia's plan to allow the US to use Colombian military bases for counter-narcotics operations. Venezuela and Ecuador strictly reject the idea while Uruguay, Chile, Argentina and Brazil dislike the agreement but respect Colombia's sovereign decisions.
Obama: "Hypocrites!"
President Barack Obama said it was hypocritical for critics of Washington's response to a coup against Honduran President Manuel Zelaya to demand a more forceful US role in returning him to power.
Zelaya, an ally of Venezuelan President Hugo Chavez, said last week the United States needs "only tighten its fist" to evict the de facto government installed after he was overthrown in June.
"The same critics who say that the United State has not intervened enough in Honduras are the same people who say we are always intervening and the Yanquis need to get out of Latin America" Obama told a closing news conference at a US-Mexico-Canada Monday summit in Guadalajara.
The issue was addressed by Obama together with Mexico's Felipe Calderón and Canadian Prime Minister Stephen Harper.
"You can't have it both ways," he insisted, without naming names. "We have been very clear in our belief that President Zelaya was removed from office illegally, that it was a coup and that he should return. We have cooperated with all the international bodies in sending that message."
Obama, Calderón y Harper expressed their full commitment "with democratic institutions and the rule of the law" in the continent and supported the Organization of American States resolutions on the Honduras case.
"We must strengthen the OAS, (Costa Rican) President Arias mediation efforts and even possibly create a Group of Friends of Honduras to help with restoring democracy," said Mexican president Calderón. He added the US must not be seen as the "great solver" for the Honduras crisis.
Latin American left has bitterly criticized Washington over the decades for intervening in the region's affairs through military force, covert action and economic pressure.
Obama told reporters in Washington last week he had no quick way to resolve the political crisis in Honduras and that the United States would not take unilateral action.
"If these critics think that it's appropriate for us to suddenly act in ways that in every other context they consider inappropriate, then I think that what that indicates is that maybe there's some hypocrisy involved in their approach to US-Latin American relations," Obama said.
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