President Donald Trump said in January that the 2015 deal between Iran and major powers must be "fixed" by May 12 or the United States will walk away.

Senior State Department official Brian Hook said on Friday after talks in Berlin and Vienna that Trump wants to reach a "supplemental" deal with the European signatories to the agreement by then.

This would cover Iran's ballistic missile programme, its regional activities, the expiration of parts of the nuclear deal in the mid-2020s and tighter UN inspections, Hook said.

"We are taking things one week at a time, we are having very good discussions in London, Paris and Berlin," Hook, recently ousted Secretary of State Rex Tillerson's chief of strategy, told reporters.

"There is a lot we agree on and where we disagree we are working to bridge our differences," Hook said.

He declined to indicate what would happen if and when such an agreement is reached, saying: "We are not under instructions from the president to go beyond seeking an agreement with our European allies."

The 2015 accord between Iran and the US, Russia, China, France, Britain and Germany curtailed Iran's nuclear activities in exchange for sanctions relief.

Iran, which according to the UN atomic watchdog has been abiding by the deal since it came into force in January 2016, has ruled out any changes to the agreement.

The talks in Vienna on Friday, a regular review of the accord, involved Iran and the six other signatories.

Trump's decision this week to replace Tillerson with Mike Pompeo as secretary of state has been widely seen as another bad omen for the agreement.

Tillerson and his erstwhile cabinet ally Defence Secretary Jim Mattis had urged Trump to listen to the Europeans to preserve the agreement.

Pompeo, head of the Central Intelligence Agency, is seen as taking a harder line on Iran.

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