The comments came as Venezuela's President Hugo Chavez denied allegations from the Pentagon that members of Iran's Revolutionary Guard Corps' Quds Force were stationed in his country.
The leftist leader, who often heaps scorn on the United States, called a recent report "absolutely false" at a military ceremony.
The head of US Southern Command, General Douglas Fraser, said Iran was bolstering its relations with Venezuela.
"We see a growing Iranian engagement with Venezuela," Fraser told reporters.
Iran has a "diplomatic, commercial presence" but, "I haven't seen any evidence of a military presence," he said.
There was no indication that Iran had sent arms to Venezuela, he added.
The general's comments came after a Pentagon report last week said Iran's Revolutionary Guards Corps had stepped up its presence in Latin America, especially in Venezuela.
Apart from its military might, the elite force — which serves under the authority of Iran's supreme leader Ali Khamenei — also wields substantial economic power and runs numerous large enterprises.
Fraser said his biggest concern was the possible "proliferation" of small arms in the region as Venezuela has bought numerous weapons from Russia.
He said Caracas had purchased 100,000 AK-103 rifles, built a factory that can produce 25,000 more and also bought up to 2,400 shoulder-launched surface-to-air missiles.
But he said there was no sign the weapons had spread beyond the Venezuelan military.
Venezuela has had tense relations with neighboring Colombia, which has accused Caracas of supporting Marxist rebels from the FARC, or Revolutionary Armed Forces of Colombia.
