The European Union's diplomatic chief Josep Borrell said Friday he was "ready" to meet Iranian leaders in Brussels as part of efforts to revive the faltering 2015 nuclear deal, but warned Tehran it was time to fully return to the negotiating table.

Wrapping up a trip to Washington, Borrell also brushed aside the notion of a "Plan B," or a possible military option as suggested this week by the United States and Israel, should Tehran fail to rejoin the accord aimed at keeping it from developing nuclear weapons.

"I know that the Iranians want to have some kind of previous talks with me as coordinator and with some members of the board of the JCPOA," Borrell told reporters, referring to the Joint Comprehensive Plan of Action, as the deal is formally known.

"I'm ready, I'm ready to do that," said Borrell. "But time is pressing."

EU envoy Enrique Mora was in Tehran Thursday to press for a firm date for resuming talks on the deal between the Islamic republic and world powers, which have stalled since June.

Tehran said following the discussions that Iran and the EU had agreed to hold further dialogue in Brussels within days.

"I cannot tell you a precise date. I am ready to receive them, if needed," said Borrell, who met a day earlier with US Secretary of State Antony Blinken, with Iran on the agenda.

"I don't say this is absolutely needed but you know I have to have a certain strategic patience on this issue, because we cannot afford to fail," he added.

'Too much at stake'

The United States, China, Russia, Germany, France and Britain reached the JCPOA agreement with Iran on its nuclear program in 2015.

Then-US president Donald Trump pulled America out of the deal in 2018, reinstating sanctions that Washington had lifted as part of the agreement.

Since then, Tehran -- which insists its nuclear program is for civilian purposes only -- has also retreated from many of its commitments under the accord.

Current US President Joe Biden has said he is ready to return to the agreement if the Islamic republic renews those commitments at the same time.

Indirect negotiations between the two foes began in April in Vienna via the other signatories to the deal but have been at a standstill since June, when a new Iranian president was elected.

Borrell said he understood that the new government in Tehran "requires time to study the file, to instruct the negotiation, but this time has been already passed. It's time to go back to the negotiation table."

Accord signatory France issued a parallel warning Friday for Iranian authorities to "urgently" put a stop to all violations of the 2015 accord, which it said were of an "unprecedented seriousness."

"Time is working against a potential agreement, because Iran is using that time to aggravate its nuclear violations, which make a return to the JCPOA ever more unlikely," said a French foreign ministry spokeswoman.

Rejecting 'Plan B'

Biden's administration hardened its tone on Iran this week -- having until now refused to consider any option other than a revival of the 2015 accord.

Blinken warned on Wednesday the United States had "other options" if diplomacy fails on Iran's nuclear program, after his visiting Israeli counterpart Yair Lapid reserved the right to use force to stop Tehran accessing atomic weapons.

But Borrell gave short shrift to suggestions of an alternative to the 2015 deal.

"I don't want to think about Plan Bs, because none of the Plan Bs that I could imagine would be a good one," he said.

"It's too important, what is at stake. The only way to prevent Iran from becoming nuclear is to go back to the deal."

Iran's Shiite clerical state has hostile relations with Israel and Saudi Arabia, although the Sunni kingdom recently entered its own talks with Tehran.

Also visiting Washington, Saudi Foreign Minister Prince Faisal bin Farhan urged Iran to return soon to talks on the nuclear deal, despite Riyadh's initial misgivings on it.

"We think we are entering a dangerous phase. So we hope these talks resume quickly," he told reporters.