"The JCPOA, it's not getting to an end," Borrell told the European Parliament after returning from a trip to the Gulf, referring to the accord formally known as the Joint Comprehensive Plan of Action.

"It would be a shame not to reach some sort of an agreement when we're so near to reaching one. But I cannot guarantee that we will reach an agreement," he said.

Iran began rolling back on most of its commitments under the accord after the United States unilaterally withdrew in 2018 and began reimposing crippling economic sanctions.

Since last year, efforts have been made to bring back the deal, with Borrell's deputy Enrique Mora leading coordination efforts.

Borrell's downbeat assessment of the JCPOA talks on Monday came after comments he made to reporters on the sidelines of the Doha Forum on the weekend, when he said that an agreement could happen in "a matter of days".

"It seems that two weeks ago, we almost had it. Then Russia came, Russia was obstructing," by withholding approval of what seemed a done deal because Moscow was looking for leverage over the West in its war in Ukraine, Borrell told MEPs in Brussels.

Specifically, he said, Russia wanted to prevent sanctions on Iranian oil being lifted "because if Iran started producing oil there will be more supply in the markets, and that's not in the interest of Russia".

After the West gave confidential guarantees related to Russian trade with Iran, Moscow lifted its objection.

But, Borrell said "now we have others related that at the end are not part of the nuclear deal -- they are collateral, like the status of the Revolutionary Guard in Iran".

That referred to Tehran demanding that Washington remove Iran's Islamic Revolutionary Guard Corps (IRGC) from a US terror blacklist.

Speaking after Mora, travelled to Iran on Sunday to try to close the gaps, Borrell said: "My teams are shuttling between Tehran, Vienna, Washington trying to find a solution."

He added: "Sometimes they think they're almost there. And other days not."

The US State Department last week said it believed a possible deal with Iran could be close. But a spokesman warned it was "neither imminent nor is it certain" -- and stressed Washington was ready to move to a "Plan B" if Tehran didn't budge.