Secretary of State John Kerry is to leave Washington Wednesday for Vienna for talks with "around a dozen countries" on the crisis in Syria.
The talks are an expanded continuation of discussions held last week in Vienna between the United States, Russia, Turkey and Syria with an expanded guest list.
State Department spokesman John Kirby said he did not know which of the powers would extend an invitation to Tehran, a key ally of Syrian leader Bashar al-Assad.
Signup to our newsletter and follow us on Facebook and Twitter!
And he said he did not know whether Tehran would accept.
"An invitation to Iran to participate, I think Iranian leaders can take to mean that it's a genuine multilateral invitation," he told reporters.
The core talks are to take place in Vienna on Friday, but diplomats expect some preparatory or bilateral meetings in the Austrian capital from late Thursday.