The United States and Iran are both warning that major disputes remain in the Vienna talks over Iran's nuclear program.
U.S. Secretary of State John Kerry said in Vienna on July 13 that "obviously we have some very significant gaps still, so we need to see if we can make some progress."
He spoke ahead of meetings on Iran with European Union foreign-policy chief Catherine Ashton and other European foreign ministers who have come to the Austrian capital to join the talks.
Kerry's cautionary remarks were echoed by the senior Iranian negotiator, Deputy Foreign Minister Abbas Araqchi.
Iranian state television quoted Araqchi as saying on July 13 that "we have not been able to narrow the gaps on major issues and it is not clear whether we can do it."
British Foreign Secretary William Hague said much the same.
"There are very significant gaps, that is very clear in these negotiations. It is unlikely that there will be a quick breakthough today," Hague said in the Austrian capital.
The remarks come as the six world powers and Iran face a July 20 deadline to complete a deal aimed at stopping Tehran from producing weapons-grade uranium.
The main sticking point is uranium enrichment, a process that can produce nuclear fuel -- Iran's stated aim -- but also material for a nuclear bomb.
A senior U.S. official present at the Vienna negotiations said on July 12 that there had been some progress in the negotiations but that on some issues Iran held some "unworkable and inadequate" positions.
The U.S. official cited as one example of such positions this week's remarks by Iran's supreme leader, Ayatollah Ali Khamenei, that Iran intends to dramatically increase the number of enrichment centrifuges it maintains from around 10,000 operational machines now to around 190,000.
The six powers want a sharp reduction, however, with Western powers saying that Iran's activities in this area should be a "fraction" of what they are now.
The senior U.S. official in Vienna also made it clear that if there is no agreement by July 20, the U.S. Congress and the president will maintain their positions regarding Iran and it is fully clear that all options will remain open.
The official told RFE/RL's Radio Farda that without considerable progress it would be very hard to renew the temporary Geneva agreement with Iran.
Iran's chief negotiator, Abbas Araqchi, warned on July 12 that Tehran was ready to walk away if "excessive" Western demands caused the talks to fail.
"If we see the excessive demands persisting and that a deal is impossible, this is not a drama, we will continue with our nuclear program," Araqchi said.
Many experts believe that if Kerry fails to get Iran to give ground he will recommend to U.S. President Barack Obama that the July 20 deadline -- when a November interim deal expires -- be pushed back.
The six world powers negotiating with Iran are the United States, Russia, China, Britain, France, and Germany.