US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif are meeting in a Vienna hotel in the first of what should be several days of negotiations ahead of a Tuesday deadline.
The US and other world powers are pursuing an agreement that would curb Iran's nuclear program for a decade in exchange for billions of dollars in sanctions relief.
Zarif says an accord is in reach unless there are "excessive demands."
He says UN sanctions must end immediately after a deal and all other penalties must be removed. The US and others say that won't happen.
