Richard Morningstar, the U.S. envoy, said in an interview with the Trend news agency in Azerbaijan that there is no chance for Iran to join the $10.7 billion project.
"I can say we have no plans regarding Iran's involvement in this project at the moment and in the future," he said.
Iran has boasted of developments in its energy infrastructure, announcing recent tenders for a pipeline to the Gulf of Oman and advancements in its Persian Pipeline. Iranian, Syrian and Turkish officials also signed a trilateral package of deals that included the development of upstream activities, drilling and exploration.
Iran's moves come amid supply concerns for Nabucco.
Europe sees the Nabucco pipeline, traveling north from Turkey to European markets, as an answer to its push for energy security and diversification.
Morningstar lauded the potential impact of the project, saying it would not only contribute to energy diversification and security, but also encourage international cooperation.
"Nabucco opens up many possibilities," he said. "We support all projects that involve the delivery of oil and gas to world markets."