The US threatened to impose 25% tariffs on cars to push Europeans to initiate proceedings against Iran for violating the nuclear deal, the German defence minister has confirmed.

“This threat exists,” said the German defence minister, Annegret Kramp-Karrenbauer, at a press conference in London.

She was asked about an article in the Washington Post that claimed Trump had secretly warned France, Germany and the UK that the US would impose a “25% tariffs on European cars” if they did not activate the mechanism for the settlement of disputes (MRD) of the Iranian international nuclear agreement reached in Vienna in 2015.

Kramp-Karrenbauer told reporters on Thursday: “This expression or threat, as you will, does exist.” She is in the UK to meet her counterpart, Ben Wallace, to discuss Anglo-European defence cooperation post-Brexit.

Her remarks came as ministers from the five countries with nationals killed in the Ukrainian plane downed by the Iranian military met in London to coordinate their response to Iran’s handling of the crash inquiry, as well as treatment of victim’s families.

Following the meeting the ministers called for an independent and transparent investigation governed by international civil aviation conventions. With emotions running high over the US assassination of the Iranian commander Qassem Suleimani, as well as the Islamic Revolutionary Guard Corps’ belated admission of responsibility for downing the jet, Iran is likely to bridle at being told how to carry out its own inquiry.

A Ukraine International Airlines Boeing 737 plane was shot down outside Tehran killing all 176 crew and passengers. Iran’s handling of the crash led to four days of street protests mainly in Tehran.

Iran initially denied responsibility for the crash, but three days later admitted that it had downed the plane believing it was an incoming US missile. An Iranian national security commission is investigating the episode.

There is concern that some in Iran are refusing to cooperate with the international investigation and refusing to hand over the black box flight recorder. There are claims within the country that the US may have jammed Iranian radar, making it impossible for the anti-aircraft battery operator to have checked the status of the plane.

Trump made his tariffs threat to Europe relatively recently and European diplomats insist they had already made the decision in principle to trigger the dispute mechanism because of previous Iranian steps away from the deal, but had not announced the move in deference to a request from China.

As a result they claim the Trump threat did not push Europe into abandoning its policy of trying to keep the nuclear deal with Iran alive.

But the threat is a further insight into Trump’s modus operandi with Europe – in effect using threats of economic sanctions and the power of the dollar to try to force Europe to follow US foreign policy.

The news will only confirm the view of the Iranian foreign minister, Javad Zarif, that Europe is failing to stand up to a high school bully. Zarif met the EU external affairs chief, Josep Borrell, to discuss the European decision to trigger the dispute mechanism, describing it as “a strategic mistake”.

Meanwhile, the Iranian president, Hassan Rouhani, revealed that Iran is now enriching more uranium than it did before it agreed to a nuclear deal with world powers in 2015.

Speaking on live TV on Thursday, Rouhani said: “We are enriching more uranium than before the deal was reached … Pressure has increased on Iran but we continue to progress.”

Iran has gradually scaled back its commitments under the nuclear deal in retaliation to Washington’s withdrawal from the pact in 2018 and its reimposition of sanctions that have crippled the country’s economy.

Trump withdrew from the nuclear agreement in part because it did not address Iran’s support for armed groups across the region and its ballistic missile programme.

Iran continued to abide by the agreement until last summer, when it began openly breaching some of its limits, saying it would not be bound by the deal if it saw none of its promised economic benefits.

After the airstrike on 3 January that killed Gen Qassem Suleimani, the architect of Iran’s regional military operations, Iran said it would abandon all restrictions in the nuclear deal.

Timeline The buildup to Qassem Suleimani's death Show Hide A rocket attack on an Iraqi military base near Kirkuk kills an American contractor and injures US and Iraqi soldiers. The US blames Shia militia group, Kata’ib Hizbullah (KH) The US conducts retaliatory airstrikes against five KH bases in Iraq and Syria, saying there had been 11 attacks against Iraqi bases hosting coalition forces in Iraq over the past two months Protesters storm the US embassy in Baghdad, trapping diplomats inside while chanting “Death to America” and slogans in support of pro-Iranian militias. At one point they breached the main gate and smashed their way into several reception rooms. The rampage was carried out with the apparent connivance of local Iraqi security forces who allowed protesters inside the highly protected Green Zone In a drone strike ordered by US president Donald Trump, the US kills Iranian general Qassem Suleimani while he was being transported from Baghdad airport

Previously it has appeared that Iran has only modestly increased its nuclear activity. In recent months it has boosted its level of enrichment of uranium to 4.5% – higher than the 3.67% limit set by the agreement but far from the 20% enrichment it was engaged in before the deal. Uranium must be enriched to 90% to be used in a nuclear weapon.

In his speech, Rouhani acknowledged the sanctions had caused economic pain but said such considerations could not be separated from foreign policy and national security.

He also acknowledged the rising tensions with the US. “A single bullet can cause a war, and not shooting a single bullet can lead to peace,” he said, adding that his administration was seeking greater security.