Pompeo is set to meet his counterparts from Britain, France and Germany
US Secretary of State Mike Pompeo arrived in Brussels on Monday to discuss the Iran nuclear deal, as Britain issued a stark warning about the risk of conflict erupting "by accident" in the Gulf.
Pompeo is set to meet 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.
British foreign minister Hunt called for "a period of calm" as he arrived for talks, warning against 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."
Alongside the meeting of all 28 foreign ministers, the representatives of Britain, France and Germany will meet Mogherini to discuss how to keep the deal going.
"We in Europe agree that this agreement is necessary for our security. No-one wants Iran to come into possession of a nuclear bomb," German Foreign Minister Heiko Maas said as he arrived.
"That is why we will continue to support the implementation of this agreement."
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.
State Department spokeswoman Morgan Ortagus said Pompeo would discuss "recent threatening actions and statements by the Islamic Republic of Iran."
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.
In recent days, Pompeo has already cancelled trips to Berlin and to Greenland to focus on the Iran issue.