US Secretary of State Mike Pompeo made a last-minute visit to Brussels where he met his counterparts from Britain, France and Germany -- the three European signatories to the 2015 accord that curbed Iran's nuclear ambitions in return for sanctions relief -- on the sidelines of a regular EU foreign ministers' gathering.

Iran last week announced it was suspending some of its commitments under the agreement, a year after US President Donald Trump withdrew from the accord and imposed swingeing sanctions on the Islamic republic -- putting the deal in peril.

Adding a military dimension to the diplomatic tensions, Washington is sending an amphibious assault ship and a Patriot missile battery to the Gulf, having already deployed an aircraft carrier and B-52 bombers.

Pompeo's visit to Brussels was announced at the last minute and if he came hoping for a show of transatlantic unity against Iran, he was disappointed, with Britain, France and Germany all publicly criticising the hardline US approach.

German Foreign Minister Heiko Maas said Berlin "still regards this nuclear agreement as the basis for Iran not having any nuclear weapons in the future and we regard this as existential for our security".

Maas said he used his one-on-one meeting with Pompeo to stress that "we are concerned about the development and the tensions in the region, that we do not want there to be a military escalation".

As he arrived for the talks, British foreign minister Hunt called for "a period of calm" and bluntly warned of the danger of pushing Iran back towards developing nuclear weapons.

"We are very worried about the risk of a conflict happening by accident with an escalation that is unintended on either side but ends with some kind of conflict," Hunt said.

"Most of all we must make sure we don't end up putting Iran back on the path to re-nuclearisation, because if Iran becomes a nuclear power its neighbours are likely to want to become nuclear powers.

"This is already the most unstable region in the world and it would be a massive step in the wrong direction."

The European Union's diplomatic chief Federica Mogherini stressed the need for dialogue as "the only and the best way to address differences and avoid escalation" in the region.

"We continue to fully support the nuclear deal with Iran, its full implementation," Mogherini said.

"It has been and continues to be for us a key element of the non-proliferation architecture both globally and in the region."

French Foreign Minister Jean-Yves Le Drian joined the criticism of the US saying Washington's move to step up sanctions against Iran "does not suit us".

- 'Nuclear bomb' -

Mogherini chaired a meeting of the so-called E3 -- Britain, France and Germany -- to discuss efforts to keep the deal going, including the special trade mechanism called INSTEX the trio set up to try to enable legitimate trade with Iran to continue without falling foul of US sanctions.

INSTEX was launched in January but is still not operational and has been dismissed scornfully by the Iranian senior leadership.

Mogherini gave a chilly response to news of Pompeo's visit, which she said was only communicated to Brussels at the last minute.

"We'll be here all day with a busy agenda so we'll see during the day how and if we manage to arrange a meeting," she told reporters.

President Hassan Rouhani issued an ultimatum to the Europeans last week threatening that Iran would go further if they fail to deliver sanctions relief to counterbalance Trump's renewed assault on the Iranian economy within 60 days.

The European powers rejected that ultimatum.

The US has continued to build pressure on Iran, with Pompeo accusing Tehran of planning "imminent" attacks and bolstering the military presence in the Gulf.

Pompeo's visit to Brussels means he is scrapping a stop expected on Monday in Moscow.

But he will still head to the Russian Black Sea resort of Sochi on Tuesday to meet President Vladimir Putin and Foreign Minister Sergei Lavrov, a State Department official added just before Pompeo left Washington.

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