"Whether they come or not is up to Iranian leaders," said U.S. state department spokesman John Kirby, according to BBC News.
"It's important for us that key partners are in these discussions... They [Iran] could be a key partner, but they are not now," he said.
Top envoys from the US, Russia, Arab and European nations are taking part in the negotiations.
Syria's Western-backed opposition and the U.S.'s Gulf Arab allies have long opposed Iran's role in the Syrian war.
Read alsoIran sends hundreds of troops to Syria in exchange for Russian S-300s - McCainMeanwhile, the U.S. said it was prepared to intensify its campaign against IS, which is also known by the acronym ISIL. The US has been carrying out air strikes against the group in Syria and Iraq since last year.
As UNIAN reported earlier, the Iranian delegation headed by Deputy Foreign Minister Hossein Amir-Abdollahian held talks in Brussels on the situation in Syria with the EU High Representative for Foreign Affairs and Security Policy Federіca Mogherini.