Tehran — German Foreign Minister Heiko Maas said Monday it was in Iran's "political and strategic interest" to stay in the 2015 nuclear deal, as the UN atomic watchdog voiced concern at increased tensions over the landmark accord.

Maas, on a visit to Tehran, also insisted that the deal -- which has been hanging in the balance since the United States unilaterally withdrew from it last year -- was "extraordinarily important" for Europe.

Iran signed the landmark accord with China, Russia, Germany, Britain, France and the United States, leading to sanctions relief in exchange for Tehran curbing its nuclear program.

But the US administration of President Donald Trump has imposed sweeping sanctions on Iran after walking away from the deal formally known as the Joint Comprehensive Plan of Action (JCPOA).

On May 8, Iran retaliated by saying it no longer considered itself bound to keep to the limits of stocks of heavy water and enriched uranium that were agreed as part of the deal.

Iran has also blamed the Europeans for not living up to their commitments.

And it warned that it would stop by early July abiding by restrictions on the level to which it can enrich uranium and on modifications to its Arak heavy water reactor, unless other parties to the JCPOA speed up work on mitigating the effects of US sanctions.

Maas met Iranian Foreign Minister Mohammad Javad Zarif Monday in Tehran to discuss the future of the nuclear deal.

"We had a serious, frank and rather long discussion," Zarif told reporters at a joint news conference after talks with Maas.

According to Maas, Germany and its European partners "have made the greatest effort to meet (their) commitments".

But Iran thinks otherwise.

"What the Europeans must do, and have done has so far, has not satisfied" our interests, Iran foreign ministry spokesman Abbas Mousavi told a news conference on Monday.

"What we expected from the European Union was that they act on their commitments, but they did not want to or could not," he added.

The US sanctions reimposed last year targeted crucial parts of Iran's economy, namely the oil and banking sector.

The oil embargo has hurt Iran's main supply of foreign revenues, while the banking sanctions scared away foreign investments and made money transfers near impossible for Iranian businesses through official channels. — Reuters