Global powers have been meeting in Vienna since early April in a bid to bring Washington back to the deal -- which the US left in 2018.

The withdrawal under then president Donald Trump and re-imposition of sanctions led to Iran stepping up its nuclear activities.

Iran in late February limited the International Atomic Energy Agency (IAEA)'s access to nuclear sites it has been monitoring as part of the 2015 deal.

A three-month agreement reached on February 21 allowing some inspections to continue was extended by another month, the IAEA announced.

"The verification and the monitoring activities that we agreed will continue as they are now for one month expiring on June 24th, 2021," IAEA director general Rafael Grossi told a news conference.

Tehran has also agreed that information collected so far by agency equipment in Iran would not be erased, he added.

- 'Not ideal' -

Grossi said the outcome of this "long discussion" was "important" but the situation was "not ideal".

"We should all be reminded that the temporary understanding is a sort of stop-gap measure. It is something that we came up with as a way to avoid flying completely blind," he said.

Iran's Supreme National Security Council said in a statement that the one-month extension aimed to "provide the needed opportunity for negotiations to progress and produce results."

Iran's envoy to the IAEA, Kazem Gharibabadi, also confirmed the agreement's extension, Iranian news agency IRNA reported.

"I advise (the countries taking part in Vienna talks) that this opportunity, which was provided over Iran's good faith, be used towards the complete lifting of sanctions in a practical and verifiable manner," he was quoted as saying.

Iran insists Washington must lift sanctions it imposed after leaving the deal before Tehran steps back its nuclear activities again.

The 2015 accord had Iran curtail its nuclear programme in return for sanctions relief.

Iranian foreign ministry spokesman Saeed Khatibzadeh said Monday that talks in Vienna depend on a "political decision" by the US, after Washington questioned Tehran's readiness to return to compliance with the accord.

"We have had very significant progress and still think that an agreement is within reach," he told reporters.

on Sunday, ahead of a fifth round of talks, US Secretary of State Antony Blinken had said it remained unclear whether Iran is "ready and willing" to take the necessary steps to return to compliance with the multi-nation nuclear agreement.

Washington and Tehran have been taking part in indirect negotiations in the Austrian capital since early April.

The EU-chaired talks also comprise Britain, China, France, Germany and Russia -- with them acting as intermediaries between Washington and Tehran.

On Wednesday EU negotiator Enrique Mora said an agreement was "shaping up" to bring the US back into the deal though there were still things that needed to be worked out.

The aim is to finish negotiations ahead of Iran's presidential elections in mid-June.