The United States is concerned by an absence of progress in nuclear talks between Iran and six world powers as "time is short" for achieving a lasting deal, a US official said Friday.
A source close to the Iranian delegation in Vienna was meanwhile quoted by the IRNA news agency as saying that "the West has to abandon its excessive demands".
The source added: "We had expected the Western side to become more realistic but this doesn't appear to be the case yet."
Both the Iranian and US side described the talks as slow and difficult.
"Discussions this week have been slow and difficult. Significant gaps remain between the two sides' positions," the senior US official in Vienna said on condition of anonymity.
"Iran still has to make some hard choices. We are concerned that progress is not being made, and that time is short," the official said.
Earlier Iran's chief negotiator also indicated that Tehran and the five permanent members of the UN Security Council plus Germany were having problems seeing eye to eye.
"It's a good atmosphere and discussions are moving forward in a spirit of goodwill, but they are moving very slowly and with difficulty," the IRNA news agency quoted Abbas Araqchi as saying.
The comments came as a fourth round of talks neared its scheduled end in the rainy Austrian capital.
Negotiators aim to nail down an exceedingly complex and lasting deal limiting Iran's nuclear programme in exchange for a lifting of sanctions before a November interim deal expires on July 20.
Failure could have calamitous consequences, potentially sparking conflict -- neither Israel nor Washington rules out military action -- and creating a nuclear arms race in the Middle East.
Negotiators could in theory extend the July 20 deadline to win more time, but Presidents Barack Obama and Hassan Rouhani could struggle to keep sceptical and impatient US and Iranian hardliners at bay.
After three rounds that Washington said helped both sides to "understand each other's positions", the United States and Iran have said they wanted to start drafting the actual agreement this time.
Even though there have been indications of some narrowing of positions, for example on the Arak reactor, both sides are sticking to the mantra that nothing is agreed until everything is agreed.
The biggest issue and main sticking point is uranium enrichment, a process making uranium suitable for peaceful uses like power generation but also, when highly enriched, for a bomb.
Multiple UN Security Council resolutions have called on Iran to suspend uranium enrichment, as has the UN atomic agency's board of governors.
The powers want to extend the time Iran would need to enrich its stockpile of low-enriched uranium to weapons-grade by slashing the number of centrifuges from the current 20,000, of which half are operating.
The Islamic republic denies wanting nuclear weapons, saying it needs the enriched uranium to fuel a fleet of nuclear reactors that it is years away from having, and other peaceful uses.
"Even with just a few thousand first generation (IR-1) centrifuges... Iran would have more than sufficient capacity for its foreseeable 'practical' nuclear power reactor fuel needs," said Daryl Kimball from the Arms Control Association.
Iran is also developing faster centrifuges that a spokesman in Tehran said Wednesday could enrich 15 times faster and which are undergoing "final mechanical testing".
Another problem issue is Iran's development of ballistic missiles, something which Tehran has said should not be part of the nuclear talks.
Washington disagrees, saying that the November deal committed Iran to address all UN Security Council resolutions, one of which -- in 2010 -- called on Iran to stop missile development.
Also to be resolved is the International Atomic Energy Agency's long-stalled probe into alleged past "military dimensions" to its programme before 2003 and possibly since.
A Thursday deadline for Iran to clear up one small part of this -- its stated need for certain detonators -- passed without comment from either the IAEA or Iran.
After a meeting on Monday, a terse IAEA statement said only that it had "noted that Iran has taken several actions and that some related work continues."