Britain, France and Germany plan a new push to keep Iran in the 2015 nuclear deal despite Tehran's threat to violate one of its central limits, but they may be nearing the end of the diplomatic road they embarked on more than 15 years ago.
The E3 countries have strained to keep the accord between major powers and Iran on life support since US President Donald Trump unilaterally withdrew from it last year and began re-imposing American sanctions.
Where the Iranians' initial response appeared to be to wait until Trump is out of office, the US president's surprise May decision to try to push Iran's oil exports to zero changed their calculus.
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The result has been a series of attacks in the Gulf that the United States blames on Iran or its proxies, despite Tehran's denials, as well as Iran's threat on Monday to breach the 2015 deal's limit on its uranium hexafluoride stocks within 10 days.
"If they do, it's essentially game over for the EU," a senior European Union diplomat said.
The attacks on six tankers in the region since the start of May, as well as two drone attacks on Saudi pumping stations, have increased fears of a US-Iranian conflict erupting by design or accident.
Diplomats said the E3, which began talking to Iran about curtailing its nuclear program in 2003, would intensify its diplomacy in the coming days, including with talks among the E3 and EU political directors in Brussels on Thursday.
The US State Department's point man on Iran, Brian Hook, plans to meet the E3 political directors in Paris on June 27, the date by which Iran says it would breach the deal, two sources said.
The British, French and German foreign ministers could visit Tehran for talks about the deal, called the Joint Comprehensive Plan of Action (JCPOA), though that is just an option for now.
And three diplomats said the Joint Commission set up under the nuclear deal could meet within the two weeks. With the US withdrawal, those talks would bring together officials from Britain, China, France, Germany, Iran, Russia and the European Union.
Diplomats stressed the E3 are weary of Iranian demands that they sustain a pact that Washington violated and said if Tehran followed suit they would have little choice but to acquiesce in the reimposition of UN sanctions.
"We need to bring them back from the brink, but let me be clear: Our margin of tolerance on the nuclear issue is zero," said a second senior European diplomat, calling for Russia and China to get more involved.
Iran said on Monday it would breach 2015 deal's 300 kilograms (660-pound) limit on its stock of uranium hexafluoride within 10 days, which the White House called "nuclear blackmail."
After Tehran's announcement, Washington said it would deploy about 1,000 more troops to the Middle East on top of a 1,500-troop increase announced following tanker attacks in May.
It remains unclear whether Iran would actually breach the deal because doing so would unite the Europeans and the Trump administration for the first time since Trump pulled out of it.
"Our assessment is that Iran still doesn't want to leave the accord," a Western intelligence source said.
An Iranian source says Tehran doubts the European nations can save the agreement.
"Almost everyone in the establishment believes that even if Europeans wanted to, they lack enough power to salvage the deal," the Iranian source said.
If Iran violated the JCPOA, there are two ways the other parties could slow any move to reimpose U.N. sanctions on top of the US sanctions already restored.
First, senior European officials have said that they will do nothing until the International Atomic Energy Agency, the Vienna-based UN nuclear watchdog charged with monitoring Iranian compliance with the deal, renders its verdict.
The next quarterly IAEA report on Iran is likely to be out on Aug. 29 or 30. However, the IAEA could also call an emergency board meeting if Iran exceeded the 300-kilogram limit.
Some diplomats suggested the E3 may not wait for an IAEA verdict and could act on their own intelligence assessments.
A second way would be to use the Joint Commission mechanism under the deal for states to complain if the accord is being breached and ultimately for sanctions to be reimposed unless the UN Security Council votes to extend relief. That entire process could take as much as 65 days.
Gerard Araud, a nuclear negotiator who recently retired as France's ambassador in Washington, made a plea for diplomacy.
"What we need in the current Iran/US tension is some diplomatic engineering," he wrote on Twitter. "Sanctioning and waiting for the other side to surrender is a recipe for failure or disaster."
