The United States says Iran is negotiating in a serious manner over its nuclear development program, but any agreement is likely to come after a Sunday deadline.
Negotiations in Vienna between U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif are still reported far apart on key issues.
Senior officials at the meeting say the size of Iran's future enrichment program remains a question. Also, they say Iran has yet to convince the U.S. it is not trying to develop nuclear weapons.
The White House on Monday left the door open to extend the talks, saying Kerry would make recommendations to President Barack Obama on how to proceed.
Kerry on Sunday said "significant gaps" remain before a deal can be reached between Iran and the U.S., Britain, China, France, Germany and Russia. An interim deal for Iran to curb its nuclear enrichment program in exchange for an easing of Western sanctions expires next Sunday.
If the deadline passes without an agreement, it would end a temporary agreement that has successfully curbed certain aspects of Iran's nuclear development program in exchange for an easing of Western economic sanctions.
Iran says its enrichment activities are aimed at producing reactor fuel, while the United States and other Western governments fear Iran is seeking to develop nuclear weaponry. The Iranian government has repeatedly denied those accusations.
Iran and the six other countries have been negotiating for months trying to agree on a comprehensive deal to ensure Iran's nuclear program is peaceful. In return, Iran wants the full repeal of international economic sanctions.