A host of barriers to the revival of Iran’s nuclear deal remain firmly in place ahead of talks due to resume this week between Tehran and world powers, suggesting a return to compliance with the 2015 accord is still a way off, four diplomats, two Iranian officials and two analysts say. 

Iranian demands about sanctions relief and Western concern over Iran's expanding nuclear know-how are among questions that may need weeks or possibly months of further negotiations, the diplomats and analysts said. 

The talks seek to revive a landmark pact under which Iran agreed to curbs on its nuclear programme in return for the lifting of international sanctions, which opened the way for a brief thaw in decades of US-Iranian confrontation. 

Then-President Donald Trump abandoned the deal in 2018 saying it was too soft on Tehran, and reimposed sanctions. Iran responded by violating the agreement's limits. 

Trump's successor Joe Biden has said he wants to restore the deal's nuclear limits and if possible extend them to cover issues such as Iran's regional behaviour and missile programme. Iran wants all sanctions lifted and no expansion of the terms. 

European Union envoy Enrique Mora, the chief coordinator of the talks, said last week he believed a deal would be reached at the upcoming sixth round of negotiations in Vienna, expected to resume on Thursday or Friday. 

Adding to the impetus to make progress is an election in Iran on June 18 to replace President Hassan Rouhani, a pragmatist who promoted the original deal. He is widely expected to be followed by a hardline successor. 

The election is not likely to change Tehran's negotiating stance: regardless of who is president any deal must be approved by Iran's hardline faction through Supreme Leader Ali Khamenei. But neither Washington nor Tehran wants to start from scratch, or entangle the deal further in Iranian domestic politics. 

Iran's top nuclear negotiator said last week that barriers to a revival of the deal are complicated but not insurmountable. "Differences have reached a point where everyone believes these differences are not insolvable," Abbas Araqchi said. 

However, none of the remaining sticking points lend themselves to rapid solutions, according to the diplomats, Iranian officials and analysts of Iranian nuclear matters. 

Their assessment chimes with downbeat remarks on Monday by US Secretary of State Antony Blinken, who said Washington still did not know whether Iran was ready to resume compliance with the deal.  

"I doubt that the next round will be the final one... The parties are still far apart on core issues," said Ali Vaez, senior Iran analyst at the International Crisis Group. 

And when, or if, disagreements are solved, more talks would be needed on so-called sequencing - the delicate question of which side takes which action and when, in return for reciprocal steps by the other side, the diplomats said. 

The negotiations have made considerable progress but are now at the hardest part with the key decisions still needed, said a European diplomat briefed on the talks, which began in April.