Efforts to revive the so-called JCPOA -- the 2015 agreement between world powers and Tehran aimed at curbing Iran's nuclear ambitions -- are at a critical juncture.

"A meeting was scheduled to take place in Vienna at the end of last week, but it was not possible. It is possible that it could take place this week," EU foreign policy chief Josep Borrell told a news conference in Santander, Spain.

Earlier this month, after more than a year of talks coordinated by Borrell and his team, the EU submitted what it called a "final" text.

The document aims at fully restoring the nuclear deal by bringing the United States back into it, after then-president Donald Trump had America withdraw in 2018.

That move prompted Iran to roll back its commitments under the JCPOA and steadily enrich its stock of uranium to close to weapons-grade levels.

Borrell said the negotiations had gone as far as they could go and "this is the inflection point".

"There was an Iranian response that I considered reasonable to transmit to the United States.

"The United States has not formally replied yet. But we are waiting for their response and I hope that response will allow us to finish the negotiation -- I hope so, but I can't assure you of it."

The other parties to the JCPOA are Britain, China, France, Germany and Russia.