The administration of United States President Donald Trump threatened to impose a 25 percent tariff on European automobile imports if the United Kingdom, France, and Germany did not formally accuse Iran of violating a 2015 nuclear deal, the Washington Post reported on Wednesday, citing unnamed European officials.

The three European countries triggered a dispute mechanism under the nuclear agreement on Tuesday, which is the strongest step they have taken so far to enforce the agreement that requires Iran to curb its nuclear programme. The mechanism amounts to a formal accusation against Tehran that it broke the terms of the deal. It could also lead to the reinstatement of United Nations sanctions that were lifted under the accord.

The European powers were all signatories to the nuclear deal. They said on Tuesday that they took the step to avoid a crisis over nuclear proliferation being added to an escalating confrontation in the Middle East.

In a statement, the three countries said that they still want the deal to succeed and that they were not joining a "maximum pressure" campaign against Iran by the US. The US abandoned the deal in 2018 and has reimposed sanctions.

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Iran has criticised that move, calling it a "strategic mistake".

Though Trump has previously made threats to slap tariffs on European automobile imports, the intent behind those threats was to receive better terms for Washington within the US-European trade relationship. This latest threat, in contrast, was intended to shift European foreign policy, according to the Washington Post.

It was not clear if the threat was necessary since the Europeans had signalled an intention to trigger the dispute mechanism for weeks, the newspaper reported.

The White House and US State Department did not immediately respond to requests for comment from Reuters News Agency.

Trump unilaterally withdrew the US from the 2015 nuclear deal - the Joint Comprehensive Plan of Action - in 2018. Washington has said its abandonment of the pact is part of a strategy intended to force Tehran back to the negotiating table to hammer out a new deal.

Iran denies its nuclear programme is aimed at building weapons. The Middle Eastern nation has gradually rolled back its commitments under the agreement since the US pulled out of the deal.

Russia, another signatory to the pact, has said it sees no grounds to trigger the dispute mechanism.