The United States is worried about increasingly close ties between Venezuela and Iran, a top US military official told lawmakers Tuesday.

"My concern, as I look at it, is the fact that there are flights between Iran and Venezuela on a weekly basis, and visas are not required for entrance into Venezuela or Bolivia or Nicaragua," said General Douglas Fraser, head of the US Southern Command.

"So we don't have a lot of visibility in who's visiting and who isn't, and that's really where I see the concerns."

Fraser said there were "growing opportunities for military-to-military connections," but that these were not apparent yet.

Last month, Washington warned Venezuelan leader Hugo Chavez against violating international sanctions against Iran over its nuclear program, which Western powers believe masks an attempt to build nuclear weapons.

US officials said last month there was no evidence the Chavez regime had violated those sanctions, but Washington is closely examining whether Venezuela's cooperation with Iran on energy issues is a violation.

Chavez and Iranian President Mahmoud Ahmadinejad both denounced American "imperialism" and called for a "new world order" during recent talks.