VIENNA - World powers and Iran appeared to make no concrete breakthrough on Friday in talks to provide Tehran with an economic package to compensate for US sanctions that begin taking effect in August.

Ministers from Britain, China, France, Germany and Russia met their Iranian counterpart in Vienna for the first time since US President Donald Trump left a nuclear accord in May, but diplomats had seen limited scope for salvaging it.

Trump pulled the United States out of the multinational deal under which sanctions on Iran were lifted in return for curbs on its nuclear program verified by the International Atomic Energy Agency (IAEA). Washington has since told countries they must stop buying the OPEC producer's oil from Nov. 4 or face financial consequences.

Speaking after three hours of talks, EU foreign policy chief Federica Mogherini, who chaired the talks, read a statement from the six delegations repeating previously-announced broad priorities ranging from guaranteeing Iranian oil revenue to shipping ties, banking and all other trade and investment co-operation.

"Participants agreed to keep progress under close review and to reconvene the joint commission, including at ministerial level, as appropriate in order to advance common efforts," Mogherini said, adding that all sides were determined to find and implement solutions.

Unlike at past meetings, Mogherini took no questions.

"All the commitments made today , should be implemented before the August deadline ... it is up to the leadership in Tehran to decide whether Iran should remain in the deal ... the proposal was not precise and a complete one," Mohammed Javad Zarif told reporters.

Speaking earlier in the day, France's foreign minister said that world powers would struggle to keep to that deadline.

"They (Iran) must stop threatening to break their commitments to the nuclear deal," Jean-Yves Le Drian said.

"We are trying to do it (economic package) before sanctions are imposed at the start of August and then the next set of sanctions in November. For August it seems a bit short, but we are trying to do it by November," he said.

On arrival in Vienna, Germany's Foreign Minister Heiko Mass said he didn't expect a collapse of talks, but suggested more negotiations would be needed in the future. He stressed hat world powers would struggle to compensate Tehran for companies leaving Iran.

The pillars of the European Union's strategy are: European Investment Bank lending, a special measure to shield EU companies from US secondary sanctions, and a Commission proposal that EU governments make direct money transfers to Iran's central bank to avoid US penalties.

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"We've made some progress, including on safeguarding some crude (oil) sales, but it's unlikely to meet Iranian expectations. It's also not just about what the Europeans can do, but also how the Chinese, Russians, Indians, others can contribute," said a senior European diplomat.

Iranian officials have said that key for them is to ensure measures that guarantee oil exports do not halt, and that Tehran still has access to the SWIFT international bank payments messaging system or an alternative.

"We are ready for all possible scenarios ... the collapse of the deal will increase the tension in the region. To save the deal, other signatories should compensate for US sanctions," a senior Iranian official told Reuters on Friday.

During a visit to Europe this week President Hassan Rouhani warned that Iran could reduce its co-operation with the UN nuclear watchdog, having already threatened Trump of the "consequences" of fresh sanctions against Iranian oil sales.

Iran's Revolutionary Guards have also warned that they may block oil shipments through the Strait of Hormuz in response to US calls to ban all Iranian oil exports.

On the nuclear side of the agreement the parties did agree to replace the United States in a working group to redesign the Arak reactor with China.

The deal specifies that the new design will aim to minimize the production of plutonium and to prevent the production of weapon-grade plutonium in normal operation. It also specifies the fuel Arak must use and says its spent fuel for the reactor's lifetime must be shipped out of Iran.

AFP adds: Russian Deputy Foreign Minister Sergei Ryabkov said this week the talks should give an "impetus" to protect the interests of economic actors.

And Chinese Foreign Minister Wang Yi said the meeting would "send a united and determined signal" to the world that the other parties "will continue to respect the agreement".

'Crime and aggression'

Iranians have complained that the hoped-for rise in foreign investment and trade after the deal has not materialised.

Since Trump's announcement, Iran's rial currency has fallen, prices have risen and the country has been hit by street protests and strikes.

Rouhani, who signed the nuclear deal, has been attacked at home by ultra-conservatives, who have denounced his willingness to talk to the West and accused him of hurting the economy.

Visiting Austria on Wednesday, he met Yukiya Amano, head of the UN nuclear watchdog IAEA which monitors Iran's compliance with the accord.

Rouhani told Amano that US sanctions were a "crime and aggression" that other nations should resist, according to IRNA.

He added that "if the other signatories, apart from the United States, can guarantee Iran's interests then Iran will stay in the JCPOA", or Joint Comprehensive Plan of Action, the accord's formal name.

Trump in May slammed the nuclear accord signed under his predecessor Barack Obama as "horrible" and "defective at its core," earning applause from Iran's regional rivals Saudi Arabia and Israel.

Iran, which strongly denies ever having sought a nuclear bomb, has warned it could resume uranium enrichment for civilian purposes if the deal collapses.

Supreme leader Ayatollah Ali Khamenei has vowed that Iran "will never tolerate both suffering from sanctions and nuclear restrictions".

US-Iran relations have been hostile since the 1979 overthrow of the US-backed shah and US embassy hostage crisis.

Washington considers Iran a state sponsor of terrorism, with links to Lebanon's Hezbollah, Hamas in the Palestinian territories and networks in Iraq and Yemen, and demands it cease support of Syrian President Bashar al-Assad.

The new round of diplomacy has been clouded after security services said they had foiled an alleged plot to bomb a Paris rally by an exiled Iranian opposition group, the People's Mujahedeen of Iran.

The large rally last Saturday attracted several US politicians, including former New York mayor Rudy Giuliani, now Trump's personal lawyer.

Six people were arrested in Belgium, France and Germany, while the opposition group blamed the Iranian regime for the alleged plot. One of those detained was a diplomat attached to the Iranian embassy in Austria.

Tehran dismissed it is an orchestrated "false flag ploy", designed to discredit Iran and overshadow Rouhani's trip to Europe, and Wednesday summoned the French and Belgian ambassadors and the German charge d'affaires in protest.