The European Union's foreign policy chief has said that there is no need to renegotiate the Iranian nuclear deal, insisting it is "delivering", despite US demands to re-open the agreement.

Federica Mogherini said all parties to the landmark 2015 accord between Iran and world powers were complying with the agreement that provides for sanctions relief in exchange for curbs on Tehran's nuclear program.

Following a UN meeting of the six powers that negotiated the deal with Iran, Ms Mogherini told reporters the United States, which has questioned the value of the deal, agreed that Iran was in full compliance with it.

She said: "There is no need to renegotiate parts of the agreement because the agreement is concerning a nuclear programme and as such is delivering."

US Secretary of State Rex Tillerson joined Iranian Foreign Minister Mohammad Javad Zarif for a first meeting with partners backing the 2015 deal.

Mr Tillerson said he had had a matter of fact exchange with his Iranian counterpart but that Washington continues to have "significant issues" with the nuclear deal.

President Donald Trump is due to report to the US Congress next month on whether he can certify that Iran is upholding its side of the accord, under which it accepted limits on its nuclear programme.

In his address to the UN General Assembly on Tuesday, Mr Trump called the nuclear deal "a disgrace" and "an embarrassment" for the United States.

But Ms Mogherini, who chaired the meeting, argued that it would be unwise to re-open the deal at a time when the world is facing a nuclear threat from North Korea.

"We already have one potential nuclear crisis. We definitely do not need to go into another one," she said.

Other than Iran and the US, the other signatories of the accord are Britain, France, Russia, China and Germany.