"I... hope that ways can be found to reduce current tensions through dialogue," International Atomic Energy Agency director general Yukiya Amano said in a speech opening the agency's quarterly board of governors meeting.

On May 8, Iran announced it no longer considered itself bound to keep to the limits of stocks of heavy water and enriched uranium which were agreed as part of the 2015 Joint Comprehensive Plan of Action (JCPOA).

Speaking later to journalists, Amano said the accord was "under tension" and confirmed that Iran's "production rate (of uranium) is increasing", although he could not give an exact figure.

Tehran's move last month came a year after US President Donald Trump pulled out of the deal. Washington has also reinforced economic sanctions against the Islamic republic.

Iran has also said that if the other parties to the JCPOA do not speed up work on mitigating the effects of US sanctions, by early July it may stop abiding by restrictions on the level to which it can enrich uranium and on modifications to its Arak heavy water reactor.

Two weeks ago, the latest inspections report by the IAEA said that while stocks of uranium and heavy water had increased, they were still within the limits set by the JCPOA.

However, Amano's speech on Monday differed from past statements because it did not explicitly say that Iran was abiding by its nuclear-related commitments under the 2015 deal.

Because of Iran's May 8 statement, "it is inappropriate for me to make the same statement of business as usual", Amano told reporters.

However, he stressed it was not the IAEA's responsibility to determine whether or not Iran was violating the JCPOA.

The latest IAEA report noted that "technical discussions... are ongoing" with Iran in relation to its installation of up to 33 advanced IR-6 centrifuges. But it did not specify the content of these discussions.

In Tehran Monday, Foreign Minister Mohammad Javad Zarif warned Monday that those waging "economic war" against Tehran through US sanctions could not expect to "remain safe".

Zarif was speaking after talks with German Foreign Minister Heiko Maas on the future of the JCPOA.

Maas acknowledged that the economic benefits Tehran hoped for from the deal were now "more difficult to obtain" but urged Iran to fully respect the agreement.

The JCPOA was struck between Iran and six major powers -- Britain, China, France, Germany, Russia and the United States -- in 2015.

Tensions between Washington and Tehran have worsened in recent weeks.

The United States has beefed up its military presence in the Middle East in response to alleged threats from the Islamic republic.