A stampede has erupted at the funeral procession of an Iranian general killed in a US air strike, killing 56 mourners, according to Iran state TV.
The burial of General Qassem Soleimani was postponed after 56 people were killed and 213 injured in the incident on Tuesday.
Hundreds of thousands of people have poured on to the streets of Iran to mourn the general after he was killed in a US drone strike in the Iraqi capital Baghdad last week.
The stampede happened during his funeral in his home town of Kerman, in south-eastern Iran. His burial has been postponed as a result of the deaths and will be held later.
The head of Iran's emergency medical services, Pirhossein Koulivand, said there had been people injured and killed.
He said: “Unfortunately, as a result of the stampede, some of our compatriots have been injured and some have been killed during the funeral processions.”
A procession in Tehran on Monday drew more than a million people in the Iranian capital, crowding both main thoroughfares and side streets.
Iran’s UN ambassador accused Mr Trump of “starting a war” through the killing of the general.
Mr Johnson is to chair a meeting of the National Security Council on Tuesday as Britain continues to urge all sides in the Gulf crisis to draw back from all-out conflict.
Foreign secretary Dominic Raab spoke on Monday to his Iranian counterpart, Mohammed Javad Zarif, to stress the need for a diplomatic resolution to avoid a renewed conflict in the region.
Addressing MPs in the Commons on Tuesday, defence secretary Ben Wallace echoed the plea and urged Tehran not to retaliate for the strike on Gen Qassem Soleimani.
He repeated earlier calls for calm, adding: “None of us wants conflict, none of us wants our citizens, our friends and our allies to be put at risk.”
Mr Raab was travelling to Brussels on Tuesday for talks with his European counterparts on the situation in the Middle East, the Foreign Office confirmed.
The so-called "E3" - Britain, Germany and France - have jointly called for "de-escalation" of the tensions between the US and Iran.
Meanwhile, Mr Johnson spoke to Turkish president Recep Tayyip Erdogan about the need to ease tensions in the Middle East, Downing Street confirmed.
Following their phone conversation, Number 10 said: “They agreed on the importance of reducing tensions and finding a diplomatic way through the current crisis, and of continuing to work together in the fight against terror.
“They agreed that Iran cannot be allowed to acquire a nuclear weapon and must comply with the terms of the nuclear deal.”
Tehran has vowed "severe revenge" for the killing of its top military commander in a US drone strike last week.
Mr Trump in turn warned that the US was ready to strike back in a "disproportionate" manner if the Iranians hit US targets.
Meanwhile, the UK’s Ministry of Defence refused to comment on reports that it is stepping up contingency plans to evacuate military and civilian personnel from neighbouring Iraq amid fears they could be targets for Iranian reprisals.
The prime minister has faced criticism that he was slow to respond to the crisis – only returning to the UK at the weekend following his new year break on the private Caribbean island of Mustique.
Mr Raab is due to fly to Washington later this week to meet secretary of state Mike Pompeo after he criticised the lack of support by the Europeans – including the UK – for the US action.
The foreign secretary echoed Mr Johnson in saying the UK would not "lament" the passing of Soleimani, whom he described as a "regional threat".
However he also warned that a threat by Mr Trump to target Iranian cultural sites would be a breach of international law.