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 U.S. 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 Trump out in hopes he would lose re-election in 2020, Trump's surprise May decision to try to push Iran's oil exports to zero has changed their calculus.
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 on condition of anonymity.
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 U.S.-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 U.S. 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 U.S. 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 U.N. 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.