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Outgoing US President Barack Obama warned against undoing the historical nuclear deal on Monday, marking the first anniversary of the pact by emphasising its "significant and concrete results".

In a statement published by the White House, Obama said "the United States must remember that this agreement was the result of years of work, and represents an agreement between the world's major powers - not simply the United States and Iran".  

It said the deal had "achieved significant, concrete results in making the United States and the world a safer place" and "verifiably prevents Iran from obtaining a nuclear weapon". 

Such a diplomatic solution, it added, was "far preferable to an unconstrained Iranian nuclear programme or another war in the Middle East".

US presidential-elect, Donald Trump - due to take office on 20 January - has been vocal in his opposition to the nuclear deal.

In a Sunday interview with The Times in the UK and Bild newspaper of Germany he continued his criticism, saying, "I'm not happy with the Iran deal, I think it's one of the worst deals ever made".

Despite this, Trump declined to say whether he intended to "renegotiate" the deal, as previously asserted regularly during the somewhat controversial presidential campaign.

On Monday, the White House insisted that despite US reservations about other actions by Iran - including its alleged support for "violent proxies" and "terrorist groups" - Tehran was upholding its nuclear commitments.

It also added that Iran had "reduced its uranium stockpile by 98 percent and removed two-thirds of its centrifuges". 

The White House stance was backed up by a statement from Secretary of State John Kerry, who helped negotiate the historic deal.

The agreement had "resolved a major nuclear threat without firing a shot or sending a single soldier into combat. It was endorsed unanimously by the United Nations Security Council and earned the support of more than 100 countries across the globe", Kerry said.

Despite Trump's sharp criticism of Iran and the nuclear deal, his policy towards the embattled nation once in office remains unclear. 

Retired marine general and one of Trump's top cabinet nominees, James Mattis said last week that if he is confirmed as defence secretary, he will support the nuclear deal.

"When America gives her word," he told senators during his confirmation hearing, "we have to live up to it and work with our allies."

The pact was signed in July 2015 by Iran and six major powers - the United States, China, Russia, Britain, France and Germany - and led to the lifting of most international sanctions against Iran exactly one year ago.