Director General of the International Atomic Energy Agency Yukiya Amano praised Iran's full commitment to its obligations under the July nuclear deal with major powers.
The pact went into effect in mid-January to place Tehran's nuclear program under time-bound constraints in exchange for relief from international sanctions.
"I noted that Iran is now subject to the strongest nuclear verification regime in the world. It is implementing not just its safeguards agreement with the agency, but also its Additional Protocol," Amano said.
"Transparency measures, which go beyond Iran's obligations under its formal agreements with the IAEA, have also been agreed."
The UN nuclear chief made the statements in a meeting of international parties to the deal on the sidelines of the Fourth Nuclear Security Summit in Washington on Friday.
About two years of negotiations between Iran and P5+1 (the five permanent members of the UN Security Council plus Germany) aimed at finding a settlement to more than 12 years of dispute over Tehran's nuclear program culminated in an agreement on July 14, 2015, that took effect six months later.
***IAEA Assurance
Under the accord, formally known as the Joint Comprehensive Plan of Action, the UN nuclear agency has been tasked with monitoring and verifying Iran's compliance.
"The IAEA has considerable experience and expertise in nuclear verification. We can effectively fulfill our responsibilities as the eyes and ears of the world in Iran and give the international community the assurances it seeks," Amano said, according to a transcript of his remarks carried by the IAEA website.
"Considerable effort was required to reach this agreement. A similar and sustained effort will be required to implement it. The IAEA will continue to do its part in a factual and impartial manner."
Iran has consistently denied its nuclear activities were aimed at developing a nuclear bomb, saying it was intended for peaceful purposes only.
***Milestone Deal
Chinese President Xi Jinping also addressed the meeting, saying the historic action plan has provided inspiration for application of dialogue and diplomacy to other major disputes.
He hailed the deal as a "milestone", calling on all parties concerned to uphold their end of the bargain.
"We should stick to the political commitment and make sure the deal is implemented as it should have been."
Xi said any "external interference" should be eliminated.
China has always been "an active participant, constructor and contributor" in the solution process, he said.
"We stand ready to work with all related sides to push forward the implementation of the deal."
