Tensions in the Middle East could pose an existential threat unless the Iran nuclear deal is maintained, Jeremy Hunt will tell EU foreign ministers in his starkest warning since the regional crisis escalated two months ago.

Arriving in Brussels on Monday, the UK foreign secretary said the agreement “isn’t dead yet”, but warned that if Tehran acquired nuclear weapons, other countries in the region would too, leading to a “very toxic and dangerous situation”.

His intervention is part of a European effort to preserve the landmark deal, which was abandoned unilaterally by the US a year ago, leading to an accelerating reciprocal withdrawal by Iran.

“Iran is still a good year away from developing a nuclear weapon,” Hunt said. “We think there is still some closing but small window to keep the deal alive.”

He said the EU was prepared to launch “a proper joint investigation” into Iranian breaches of the deal, following Tehran’s recent announcements that it would start enriching uranium beyond agreed limits. “What we are looking for is to give Iran a way out of this so that they can get back into compliance with the nuclear deal,” he said.

Q&A What is Iran's nuclear enrichment cap? Show Hide Under the joint comprehensive plan of action signed in 2015, Iran is only permitted to produce low-enriched uranium. This is uranium that has a concentration of 3%-4% of the isotope U-235. This isotype is a fissile material, which is capable of sustaining a nuclear fission chain reaction. At this low level of enrichment, it can be used to produce fuel for nuclear power plants. Iran is permitted to stockpile 300kg of it, at an enrichment level capped at 3.67%. A bomb needs uranium that is up to 90% enriched. Iran’s atomic energy agency has said production of enriched uranium has been quadrupled. The more that uranium is enriched, the faster it is to enrich it further. Iran argues that it is no longer bound by the cap because of the withdrawal of the US from the agreement, and the reimposition of economic sanctions Donald Trump.

Read more: Iran's uranium enrichment programme: the science explained

The foreign secretary, who is trailing Boris Johnson in the race to become UK prime minister, downplayed a rift with the US over policy on Iran. Those differences were manifest in leaked cables of the outgoing British ambassador to Washington, Sir Kim Darroch, who wrote that the US president, Donald Trump, scuppered the deal to spite his predecessor, Barack Obama, in an act of “diplomatic vandalism”.

“We have different views as to how we keep the region denuclearised, but we have very open channels of communication on that,” Hunt said.

Hunt has tried to de-escalate the situation by saying an Iranian-owned oil tanker seized by the British off Gibraltar 10 days ago might be released if Tehran promised the ship’s owners would abandon plans to unload its oil in Syria. The EU has imposed a ban on oil sales to Damascus.

Hunt made the offer in a phone call with the Iranian foreign minister, Mohammad Javad Zarif, on Saturday. But Zarif said afterwards that Iran should be entitled to sell oil to any country it wished, and claimed the UK seizure of the ship, Grace 1, was an act of piracy. Various Iranian government sources said the tanker was not bound for Syria, but Tehran is desperate for revenue from oil sales.

Hunt is due to tell his European counterparts: “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.”

Jeremy Hunt (@Jeremy_Hunt) Heading to Brussels for urgent talks on how to reduce tensions with Iran. Their approach to Mid East has been profoundly destabilising but we want to reduce not raise tensions over Grace 1 and avoid a nuclearised region. But a deal is a deal and if one side breaches it... pic.twitter.com/n9MngbpZuX

France, Germany and the UK – the three European signatories to the nuclear deal – issued another joint statement ahead of the meeting urging Iran to come back into compliance.

“The risks are such that it is necessary for all stakeholders to pause and consider the possible consequences of their actions,” the statement said. “We believe that the time has come to act responsibly and to look for ways to stop the escalation of tension and resume dialogue.

“We are concerned by the risk that the nuclear deal further unravels under the strain of sanctions imposed by the United States and following Iran’s decision to no longer implement several of the central provisions of the agreement.”

Iran first breached the nuclear deal by overstepping the agreed limits on its low enriched uranium stockpile and then by surpassing the level at which the uranium can be enriched. Neither step in itself puts Iran close to securing a nuclear bomb.

The EU has yet to put the two Iranian decisions into the dispute mechanism of the joint comprehensive plan of action, as the nuclear deal signed in 2015 is known, but it must be close to doing so.

In a repetition of Tehran’s position, the Iranian president, Hassan Rouhani, said in a televised speech on Sunday: “We have always believed in talks … if they lift sanctions, end the imposed economic pressure and return to the deal.”

Zarif is due to visit New York this week for a meeting at the UN, even though a fortnight ago the US said it would be imposing sanctions on him personally, a threat Washington has so far not carried out. It is understood he has been granted a US visa to travel to New York.

Zarif gave no hope of an early release for the British-Iranian dual national Nazanin Zaghari-Ratcliffe, saying: “The arrest and trial of these individuals were based on completely legal procedures, and the British government is expected to respect the rule of law and the independence of the judiciary of the Islamic Republic of Iran.”