VIENNA - A fourth round of nuclear talks between Iran and six world powers ended Friday with both sides complaining that major gaps remained ahead of a July 20 deadline for a vaunted accord.

Iran’s chief nuclear negotiator Abbas Araqchi quoted on state television, reported “no tangible progress” at the talks in Vienna as he described the gaps as “too huge”.

However, he said Tehran remained “determined” to continue the talks in coming months.

Separately, a Western diplomat said “huge gaps” remain in the negotiations aimed at finding a lasting deal on limiting Tehran’s nuclear programme, and called on Iran to show more flexibility.

“Huge gaps remain, there is really more realism needed on the other side,” the diplomat said. “We had expected a little more flexibility on their side.” Unusually, no press conference was held and no statement issued after the three-day meetings between Iran and the so-called P5+1 powers — the five permanent members of the UN Security Council plus Germany — bolstering the impression that little had been achieved.

A US official had earlier said Washington was worried by a lack of progress in the talks, calling the discussions “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.

Michael Mann, spokesman for EU foreign policy chief and the six powers’ lead negotiator Catherine Ashton, however warned against reading too much into the lack of a joint statement or press conferences after the meetings.

“Nothing is agreed until everything is agreed. That’s why we don’t want to break things down and give a snapshot of where we are after every session,” he told AFP.

“We have had three days of hard work. As we have said, the negotiations are complex and detailed,” he said.

“We are trying to formulate an agreement.”

Both the Iranian and US side said the parties were having problems seeing eye to eye.

A source close to the Iranian delegation in Vienna was quoted by the IRNA news agency as saying that “the West has to abandon its excessive demands”.

“We had expected the Western side to become more realistic but this doesn’t appear to be the case yet,” the source added.

On Friday evening, a US official said “there needs to be some additional realism,” admitting “moments of great difficulty” in the talks.

A next round of Iran talks was set for June with the EU still to fix the date, the official said.

- Drafting an actual deal -

Negotiators are trying to nail down an exceedingly complex and lasting deal that would curb Iran’s nuclear programme in exchange for a lifting of sanctions before a November interim agreement 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 US President Barack Obama and his Iranian counterpart Hassan Rouhani could struggle to keep sceptical and impatient hardliners from their respective countries at bay.

After three rounds that Washington said helped both sides to “understand each other’s positions”, Washington and Tehran had said they wanted to start drafting the actual agreement this time.

That however proved elusive.

Even with 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, which can make the element suitable for power generation but also, when highly enriched, for a bomb.

Multiple UN Security Council resolutions have called on Iran to suspend this process, as has the International Atomic Energy 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 and says enrichment is only for peaceful uses.

- Ballistic missiles -

Another issue is Iran’s development of ballistic missiles, a point 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 IAEA’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.”