By John Irish and Parisa Hafezi

PARIS/ISTANBUL (Reuters) -In a bid to defuse the conflict over Iran's nuclear program, foreign ministers from Europe's top three powers hurried to meet their Iranian counterpart on Friday in Geneva.

Those hopes collapsed on Saturday when U.S. President Donald Trump ordered airstrikes on Iran's three main nuclear sites, in support of Israel's military campaign.

"It's irrelevant to ask Iran to return to diplomacy," Iran's foreign minister Abbas Araqchi, visibly angry, told reporters in Istanbul on Sunday, promising a "response" to the U.S. strikes. "It's not time for diplomacy now."

Trump, who said the U.S. airstrikes "obliterated" the sites, warned in a televised speech on Saturday the U.S. could attack other targets in Iran if no peace deal was reached and urged Tehran to return to the negotiating table.

Reuters spoke to seven Western diplomats and analysts who said the prospect of negotiations was negligeable for now, with an unbridgeable gap between Washington's demand for zero enrichment by Iran and Tehran's refusal to abandon its nuclear program.

"I think the prospects of effective diplomacy at this point are slim to none," said James Acton, co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace, a think tank headquartered in Washington.

"I'm much more worried about escalation, both in the short and the long term."

According to European diplomats, the three European allies - Britain, France and Germany - were not made aware of Trump's decision to strike Iran ahead of time. French President Emmanuel Macron had promised on Saturday - just before the U.S. strikes - to accelerate the nuclear talks, following a call with his Iranian counterpart.

One European diplomat, who asked not to be identified, acknowledged there was now no way of holding a planned second meeting with Iran in the coming week.

In the wake of the U.S. military action, any European diplomatic role appears likely to be secondary. Trump on Friday dismissed Europe's efforts towards resolving the crisis, saying Iran only wanted to speak to the United States.

Three diplomats and analysts said any future talks between Iran and Washington would likely be through regional intermediaries Oman and Qatar, once Tehran decides how to respond to the U.S. airstrikes on its nuclear sites at Fordow, Natanz and Isfahan.

The attacks leave Iran with few palatable options on the table. Since Israel began its military campaign against Iran on June 13, some in Tehran have raised the prospect of withdrawing from the Nuclear Non-Proliferation Treaty (NPT) to signal Iran's determination to accelerate enrichment, but experts say that would represent a considerable escalation and likely draw a forceful response from Washington.

Acton, of the Carnegie Endowment, said Iran's most obvious means for retaliation is its short-range ballistic missiles, that could be used to target U.S. forces and assets in the region. But any military response by Iran carried the risk of miscalculation, he said.

"On the one hand, they want a strong enough response that they feel the U.S. has actually paid a price. On the other hand, they don't want to encourage further escalation," he said.

EUROPEAN EFFORT ENDED IN FAILURE

Even before the U.S. strikes, Friday's talks in Geneva showed little sign of progress amid a chasm between the two sides and in the end no detailed proposals were put forward, three diplomats said. Mixed messaging may have also undermined their own efforts, diplomats said.

European positions on key issues like Iran's enrichment program have hardened in the past 10 days with the Israeli strikes and the looming threat of U.S. bombing.

The three European powers, known as the E3, were parties to a 2015 nuclear deal that Trump abandoned three years later during his first term.

Both the Europeans and Tehran believed they had a better understanding of how to get a realistic deal given the E3 have been dealing with Iran's nuclear programme since 2003.

But the Europeans have had a difficult relationship with Iran in recent months as they sought to pressure it over its ballistic missiles programme, support for Russia and detention of European citizens.

France, which was the keenest to pursue negotiations, has in the last few days suggested Iran should move towards zero enrichment, which until now was not an E3 demand given Iran's red line on the issue, two European diplomats said.

Britain has also adopted a tougher stance more in tune with Washington and that was expressed in Geneva, the diplomats said. And Germany's new government appeared to go in the same direction, although it was more nuanced.

"Iran has to accept zero enrichment eventually," said one EU official.

A senior Iranian official on Saturday showed disappointment at the Europeans' new stance, saying their demands were "unrealistic", without providing further details.

In a brief joint statement on Sunday, which acknowledged the U.S. strikes, the European countries said they would continue their diplomatic efforts.

"We call upon Iran to engage in negotiations leading to an agreement that addresses all concerns associated with its nuclear program," it said, adding the Europeans stood ready to contribute "in coordination with all parties".

David Khalfa, co-founder of the Atlantic Middle East Forum, a Paris-based think tank, said Supreme Leader Ali Khamenei's government had taken advantage of the Europeans for years to gain time as it developed its nuclear program and ballistic missile capabilities.

"The European attempt ended in failure," he said.

However, the Europeans still have one important card to play. They are the only ones who, as party to the nuclear accord, can launch its so-called "snapback mechanism", which would reimpose all previous UN sanctions on Iran if it is found to be in violation of the agreement's terms.

Diplomats said, prior to the U.S. strikes, the three countries had discussed an end-August deadline to activate it as part of a 'maximum pressure' campaign on Tehran.

"MULTIPLE CHANNELS" FOR U.S. TALKS

In total, the U.S. launched 75 precision-guided munitions, including more than two dozen Tomahawk missiles, and more than 125 military aircraft in the operation against the three nuclear sites, U.S. officials said.

US Defense Secretary Pete Hegseth on Sunday warned Iran against retaliation and said both public and private messages had been sent to Iran "in multiple channels, giving them every opportunity to come to the table."

Five previous rounds of indirect negotiations between the United States and Iran collapsed after a U.S. proposal at the end of May called for Iran to abandon uranium enrichment. It was rejected by Tehran, leading to Israel launching its attack on Iran after Trump's 60-day deadline for talks had expired.

Iran has repeatedly said from then on that it would not negotiate while at war.

Even after Israel struck, Washington reached out to Iran to resume negotiations, including offering a meeting between the Trump and Iranian President Masoud Pezeshkian in Istanbul, according to two European diplomats and an Iranian official.

That was rebuffed by Iran, but Araqchi did continue direct contacts with US Special envoy Steve Witkoff, three diplomats told Reuters.

One of the challenges in engaging with Iran, experts say, is that no-one can be sure of the extent of the damage to its nuclear program. With the IAEA severely restricted in its access to Iranian sites, it is unclear whether Tehran has hidden enrichment facilities.

A senior Iranian source told Reuters on Sunday most of the highly enriched uranium at Fordow, the site producing the bulk of Iran's uranium refined to up to 60%, had been moved to an undisclosed location before the U.S. attack there.

Acton, of the Carnegie Endowment, said that - putting aside from the damage to its physical installations - Iran had thousands of scientists and technicians involved in the enrichment program, most of whom had survived the U.S. and Israeli attacks.

"You can't bomb knowledge," said Acton.

(Additional reporting by Lili Bayer and Andrew Gray in Brussels, and Tom Perry in Beirut; Editing by Daniel Flynn)

Posted by: AAngeles