A U.S. State Department official says significant gaps remain in negotiating positions on Iran's uranium enrichment program.
But the official said Iran and global powers are not discussing an extension of their November 24 deadline for a deal to eliminate concerns Tehran could develop nuclear weapons.
The official spoke ahead of a meeting on October 15 between U.S. Secretary of State John Kerry, Iranian Foreign Minister Mohammad Javad Zarif and EU foreign policy chief Catherine Ashton in Vienna.
"We're not talking about extension or anything like that in the room. We're talking about getting this done by the 24th (of November)," the official said.
But the official said "we very well may not" reach a deal by the deadline.
An accord would curtail Iranian activities including uranium enrichment in exchange for the removal of sanctions.
In Geneva on October 15, Iranian Parliament Speaker Ali Larijani said U.S. nuclear negotiators should stop focusing on the number of enrichment centerfuges Iran may have, calling it a "trivial matter".