British foreign secretary Jeremy Hunt is heading to Brussels today to seek support from France and Germany over Iran.
The three countries fear the Iran nuclear deal is close to collapse after the US pulled out last year and reimposed sanctions which have antagonised the Iranians.
Tehran has been stockpiling more low-enriched uranium than the deal permits and has also started to enrich uranium above the 3.67% allowed.
The agreement struck in 2015 had required Iran to get rid of its medium-enriched uranium, cut its stocks of low-enriched uranium and reduce the number of its gas centrifuges for 13 years.
In exchange, sanctions would be lifted.
Despite the US leaving the deal, European countries have been trying to save it and, ahead of his departure, Mr Hunt said: "The Middle East is already one of the most unstable regions in the world, but if the different parties were armed with nuclear weapons it would represent an existential threat to mankind.
"I will do everything in my power to prevent that from happening.
"I'll be building on the leadership shown by the UK, alongside France and Germany, as we do what it takes to maintain the nuclear deal, and to work to encourage Iran back into compliance."
On Sunday, Britain, France and Germany said in a statement that they were "deeply troubled" by the recent tensions in the region.
"We believe the time has come to act responsibly and seek a path to stop the escalation of tensions and resume dialogue," they added.
"The risks are such that it is necessary for all stakeholders to pause and consider the possible consequences of their actions."
The latest developments come after the Royal Marines were involved in the seizure of an oil tanker off Gibraltar earlier this month.
The ship had been suspected of taking oil to Syria, in breach of EU sanctions.
Iran has demanded the release of the supertanker and has threatened reprisals.
Mr Hunt said he and Iran's foreign minister Mohammad Javad Zarif discussed the crisis by phone on Saturday, and on Sunday he said it was important to maintain freedom of navigation in the Gulf.
"Following the brave work of our Royal Navy in the Persian Gulf we will work with partners to maintain the right of commercial ships to go about their rightful business," he said.
Mr Zarif will be at a UN meeting in New York this week but it is not clear whether the visit might also be used to discuss the nuclear issue.
President Hassan Rouhani said on Sunday Iran was ready to talk to the US if Washington lifted sanctions and returned to the 2015 nuclear deal.
In a televised speech, Mr Rouhani said: "We have always believed in talks... if they lift sanctions, end the imposed economic pressure and return to the deal, we are ready to hold talks with America today, right now and anywhere."
