Iran and world powers resumed talks on Monday after a five-month hiatus to try to salvage their 2015 nuclear deal but with Tehran sticking to its tough stance and Western powers warning that will not work, hopes of a breakthrough appeared slim.

Diplomats say time is running out to resurrect the pact, which then-U.S. President Donald Trump abandoned in 2018, angering Iran and dismaying the other powers involved - Britain, China, France, Germany and Russia.

Six rounds of indirect talks were held between April and June. The new round formally began with a meeting of the remaining parties to the deal, without the United States, shortly after 1400 GMT.

The meeting in Vienna ended an extended break triggered by the election of hardline cleric Ebrahim Raisi in June as Iran's president. The talks are effectively indirect negotiations between Tehran and Washington since Iran refuses to meet face to face with U.S. envoys. Other officials shuttle between them.

Tehran's negotiating team has set out demands that U.S. and European diplomats consider unrealistic, Western diplomats say.

"Our demands are clear. Other parties and especially Americans should decide whether they want this deal to be revived or not. They abandoned the pact, so they should return to it and lift all sanctions," an Iranian official close to the talks told Reuters.

Iran has adopted an uncompromising position by demanding removal of all U.S. and European Union sanctions imposed since 2017, including those unrelated to its nuclear programme, in a verifiable process. read more

"The United States still fails to properly understand the fact that there is no way to return to the deal without a verifiable and effective lifting of all sanctions," Iranian Foreign Minister Hossein Amirabdollahian said in a statement shortly after the talks resumed. 

"The return of the U.S. to the nuclear deal would be meaningless without guarantees to prevent the recurrence of the bitter experience of the past," he said. "This opportunity is not a window that can remain open forever."

TENSIONS

In parallel, Tehran's conflicts with the U.N. atomic watchdog, which monitors its nuclear programme, have festered.

As Iran has advanced its uranium enrichment, the International Atomic Energy Agency says its inspectors have been treated roughly and refused access to reinstall monitoring cameras at a site it deems essential to reviving the deal.

"If Iran thinks it can use this time to build more leverage and then come back and say they want something better, it simply won't work. We and our partners won't go for it," U.S. envoy Robert Malley told BBC Sounds on Saturday.

Since Trump took the United States out of the deal, Iran has breached many of its restrictions meant to lengthen the time it would need to generate enough fissile material for a nuclear bomb. Iran says it wants to enrich uranium only for civil uses.

Malley warned that Washington would be ready to ramp up pressure on Tehran if the talks collapse. 

Diplomats have said Washington has suggested negotiating an open-ended interim accord with Tehran as long as a permanent deal is not achieved. Several Iranian officials told Reuters Iran had no intention of accepting an interim deal.

Iran's arch-enemy Israel, which opposed the original deal as too limited in scope and duration, has said military options will be on the table if diplomacy fails.

"They (Iranians) will play for time, earn billions from the removal of sanctions, continue to deceive the world, and covertly advance their nuclear programme," Israeli Foreign Minister Yair Lapid told reporters in London.

Reuters