International Atomic Energy Agency (IAEA) Director General Yukiya Amano has traveled to the United States to discuss Iran's nuclear agreement with officials from the administration of President Donald Trump, who has denounced it as "the worst deal ever negotiated."

Secretary of State Rex Tillerson held private talks with the Japanese director of the IAEA at the State Department in Washington, DC on Thursday.

Media reports said other senior US officials were present during the talks, but there have been no details of the discussions yet.

This is the first official meeting between the chief of the United Nations nuclear watchdog monitoring the Iran nuclear deal and senior officials from the new US administration.

Trump has been sharply critical of the 2015 agreement known as the Joint Comprehensive Plan of Action (JCPOA).

During his election campaign, Trump had promised to either “tear up” the accord or renegotiate its terms if elected president.

Tillerson has also called for a "full review" of the nuclear deal.

Prior to his meeting with Tillerson, Amano said the UN body was in "constant interaction" with US civil servants.

The IAEA chief has publicly defended the landmark nuclear agreement, saying the UN atomic agency is making efforts to help implement the agreement signed between Iran and the P5+1 group of countries.

“Implementation is very important,” he emphasized, adding, “There is nothing political that will change our implementation."

Officials from Iran, the EU, and the P5+1 group of countries pose for a picture during a final press conference after the Iran nuclear talks in Vienna, Austria, July 14, 2015. (Photo by AFP)

Iran and the P5+1 group - the five permanent members of the UN Security Council, namely the US, France, Britain, Russia and China, plus Germany - started implementation of the JCPOA on January 16, 2016.

The deal, which was later approved in a resolution by the UN Security Council, lifted sanctions imposed on Iran, which, in turn, put some limits on its nuclear program.

In a joint statement last month, the European Union (EU) and the IAEA reaffirmed their support for the nuclear agreement.