US President Donald Trump was set Friday to decide whether to jeopardize the 2015 Iran nuclear deal by re-imposing American sanctions, against the wishes of European leaders who say the agreement is working just fine.

Washington agreed to sanctions relief under the terms of the landmark accord reached between Tehran and six world powers -- which Trump has denounced as one of the worst deals of all time.US officials expect the White House to issue a statement later Friday to confirm that Trump has grudgingly signed the sanction waivers once again before they start to expire this weekend -- though he is also expected to impose new sanctions on Tehran over non-nuclear issues.

But the angst in Berlin, Brussels, London and Paris underlined what is at stake: the mercurial US president is quite capable of sabotaging the agreement.

America's allies see the accord as the best way of thwarting Iran's quest for nuclear arms and a victory for multilateral diplomacy. Tehran categorically denies it is seeking to develop atomic weapons.

But Trump argues that his predecessor Barack Obama gave away too much to Iran in sanctions relief, without forcing the Islamic republic to end its ballistic missile program and aggressive support for militant groups.

"The president still strongly believes this is one of the worst deals of all time," White House spokeswoman Sarah Sanders told reporters on Thursday as Trump met with his top national security aides on Iran.

"One of the single greatest flaws is its restrictions leave Iran free... to openly develop their nuclear program and rapidly achieve a nuclear weapons breakout capability."

- Sanctions relief -

Trump has already declared that he thinks the Iran nuclear deal is no longer in the United States' national interest.

By thus "decertifying" the arrangement, he opened a window for Congress to reimpose sanctions, but to date, it has not done so -- leaving the issue of the waivers.

So far, Trump has continued to follow Obama's lead in regularly signing sanctions waivers so that US economic measures against Tehran do not "snap back."

The deadlines for a number of these waivers to be renewed will fall over the coming week, and Trump now is obliged to decide whether to maintain sanctions relief.

If he does allow the punitive measures to go back into effect, Iran will accuse the United States of breaking the deal, under which Tehran accepted restrictions on its nuclear program.

European capitals will also be dismayed, having pressed Washington to accept that the deal was an international agreement and that Iran has abided by its terms.

Russia spoke out strongly Friday in favor of the accord, ahead of the announcement of Trump's decision, calling it "the result of a consensus among many parties," in the words of Kremlin spokesman Dmitry Peskov.

The accord, he added, can be considered "either good or bad, but it is the only one that reflects this consensus," Peskov added at a press conference.

French President Emmanuel Macron called Trump on Thursday and stressed France's determination to see "the strict application of the deal and the importance of all the signatories to respect it."

The White House said Trump had "underscored that Iran must stop its destabilizing activity in the region."

In any event, the US is likely to impose new sanctions on Iran over human right abuses and support for foreign extremist groups rather than nuclear back-sliding.

"I think you can expect there will be more sanctions coming up," Treasury Secretary Steven Mnuchin told reporters on Thursday.

- Arms race -

In Brussels on Thursday, the European Union and the foreign ministers of Britain, Germany and France presented a united front after talks with Iranian Foreign Minister Mohammad Javad Zarif.

"The deal is working, it is delivering on its main goal which means keeping the Iranian nuclear program in check," EU foreign policy chief Federica Mogherini said.

The agreement, she said, is "making the world safer and... preventing a potential nuclear arms race in the region."

UN inspectors have certified Iran's compliance with the deal nine times, most recently in November.

Iran has said that if the US walks away from the agreement, it is ready to give an "appropriate and heavy response".

Zarif took to Twitter after the Brussels meeting to warn that "Iran's continued compliance (is) conditioned on full compliance by the US."

One of the criticisms levelled at the nuclear deal is that it does nothing to address Iran's continuing ballistic missile program and involvement in conflicts such as Yemen and Syria.

Europeans say these issues should be kept separate from discussion of the deal, but in a nod to US concerns, Mogherini stressed they were raised with Zarif on Thursday.