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 on 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.