Guterres "encourages Iran to continue to implement its nuclear-related commitments and calls on all participants to abide fully by their respective commitments," his spokesman Stephane Dujarric said.

The UN chief also urged "other member states to support the plan's implementation."

"The secretary-general urges all parties to refrain from any steps that may lead to further escalation of tensions in the region," the spokesman added.

Iran said Monday that as of June 27, it will have more than the 300 kilos (660 pounds) of enriched uranium that it was allowed to have under the deal originally reached with Britain, China, France, Germany, Russia and the United States.

Last year, US President Donald Trump abandoned the deal, under which Iran pledged to reduce its nuclear capacities for several years and allow in inspectors in exchange for sanctions relief.

Washington then unilaterally reimposed crippling sanctions on Tehran.

Guterres said the agreement "represents a major achievement in nuclear non-proliferation and diplomacy" and has "contributed to regional and international peace and security," the spokesman said.