"Preserving the deal is not only our interest and our responsibility – it is – but it is also a joint responsibility for all around the world," Federica Mogherini told the plenary session of the European Parliament in Strasbourg, France.
“This is our interest, this is Iran's interest, this is our collective interest… That kind of the deal was in the interest of all of us, collectively, and I would say in the interest of collective security in the world,” she said.
Mogherini made the comments after the International Atomic Energy Agency (IAEA) confirmed last week said Iran is now enriching uranium to 4.5% fissile purity, above the 3.67% limit set by its deal, the second major move in as many weeks after Tehran exceeded limits on its stock of low-enriched uranium.
The level at which Iran is now refining uranium is still well below the 20% purity of enrichment Iran reached before the deal. Low-enriched uranium provides fuel for civilian power plants.
Iran rolled back some of its commitments to the JCPOA more than a year after the US pulled out of it and reinstated sanctions which were lifted under the multilateral agreement.
US sanctions have notably targeted Iran’s vital oil revenue stream.
Iran says the European countries must do more to guarantee it the trade and investment dividends it was due to receive in return for UN-monitored limits to its nuclear capacity under the deal.
The Europeans are also trying to set up INSTEX, a barter-based trade conduit with Iran, but an equivalent Iranian mechanism has yet to start. Should the mechanism go ahead it would initially only deal in products such as pharmaceuticals and foods, which are not subject to US sanctions.
Mogherini said Iran's actions “are still reversible” and do not indicate an "unequivocal decision or intention" revert to its nuclear program.
"At the same time, I want to be very clear, the longer Iran proceeds down this road, the harder it will be, technically, to step back. Our request to Iran is very clear: Go back to the full implementation of the deal as you have been doing so far," she added.
On Monday, Mogherini said the remaining parties to the nuclear deal do not see Iran’s scaling down of its commitments as significant and do not intend for now to trigger the pact’s dispute mechanism, preferring more diplomacy to ease the crisis.
Under the terms of the deal, if any party believes another is not upholding their commitments they can refer the issue to a Joint Commission comprising Iran, Russia, China, Britain, France, Germany and the European Union.
This activates a dispute mechanism that could eventually end with a restoration of global, UN sanctions against Iran. Mogherini said a Joint Commission meeting was possible, although when and at what level had yet to be decided.
Reuters contributed to this report.