CARACAS, Venezuela — A plan to increase the American military presence on at least three military bases in Colombia, Washington’s top ally in Latin America, is accentuating Colombia’s already tense relations with some of its neighbors.

Venezuela, Ecuador and Nicaragua, which are members of a leftist political alliance that is led by President Hugo Chávez of Venezuela and backed by his nation’s oil revenues, have all criticized the plan, saying it would broaden the military reach of the United States in the Andes and the Caribbean at a time when they are still wary of American influence in the region.

Despite a slight improvement in Venezuela’s relations with the United States in recent months, Mr. Chávez has been especially vocal in lashing out at the plan. Speaking on state television here Monday night, he put Venezuela’s diplomatic ties with Colombia under review, calling the plan a platform for “new aggression against us.”

Colombia’s foreign minister, Jaime Bermúdez, on Tuesday defended the negotiations, which are expected to produce an agreement in August, asking neighboring countries not to interfere in Colombia’s affairs. “We never expressed our opinion in what our neighbors do,” he said, pointing to Mr. Chávez’s attempts to strengthen ties with non-Western nations. “Not even when the Russian presence became known in Venezuelan waters, or with relations with China,” he added.

The United States has been negotiating the increase of military operations in Colombia in recent weeks, faced with Ecuador’s decision to end a decade-long agreement allowing E-3 AWACs and P-3 Orion surveillance planes to operate from the Manta Air Base on Ecuador’s Pacific Coast.

Preliminary terms of the agreement would shift some operations to Palanquero, a military base near Bogotá, Apiay Air Base on Colombia’s eastern plains and Alberto Pouwels Air Base near Barranquilla on the Caribbean coast, according tonews reports. In addition, American warships would be able to use two Colombian naval bases, Bahia de Málaga near Buenaventura on the Pacific, and another on the Caribbean in the department, or province, of Bolívar.

While American antidrug surveillance flights would sharply increase in Colombia, the world’s top producer of cocaine, the agreement would not allow American personnel to take part in combat operations in the country, which is mired in a four-decade war against guerrillas. A limit of 800 American military personnel and 600 American military contractors would also remain in place, officials involved in the talks said.

Still, depending on how the accord is put in place, American troop levels in Colombia could climb sharply. The United States currently has about 250 military personnel in the country, deployed largely in an advisory capacity to Colombia’s armed forces, William Brownfield, the United States ambassador to Colombia, said last week.

Colombia, which has already received more than $5 billion in military and antidrug aid from the United States this decade, has found itself isolated diplomatically as Mr. Chávez presses ahead with his efforts to expand Venezuela’s oil diplomacy while eroding American influence in the hemisphere.

Other countries chafe at Colombia for different reasons. Colombia has not had diplomatic relations with Ecuador since Colombian forces carried out a raid on a Revolutionary Armed Forces of Colombia, or FARC, rebel camp on Ecuadoran territory last year. A festering boundary dispute with Nicaragua has also made for tensions between Colombia and Nicaragua’s president, Daniel Ortega, an ally of Mr. Chávez.

But with Venezuela itself, Colombia remains locked in a complex game of interdependence.

Its sales of manufactured and agricultural goods to Venezuela remain resilient despite Mr. Chávez’s occasional outbursts directed at his ideological opposite, Colombia’s president, Álvaro Uribe. And faced with disarray in its oil industry, Venezuela relies on imports of Colombian natural gas, narrowing the possibility of a severe deterioration in ties between the two countries despite their sharply different views of cooperation with the United States.