"President Raisi's helicopter was completely burned in the crash ... unfortunately, all passengers are feared dead," the official told Reuters.

Rescue teams fought blizzards and difficult terrain through the night to reach the wreckage in East Azerbaijan province in the early hours of Monday.

“We can see the wreckage and the situation does not look good,” the head of Iran’s Red Crescent, Pirhossein Kolivand, told state TV.

Raisi, 63, was elected president in 2021, and since taking office has ordered a tightening of morality laws, overseen a bloody crackdown on anti-government protests and pushed hard in nuclear talks with world powers.

A Turkish drone identified a source of heat suspected to be the helicopter's wreckage and had shared the coordinates of the possible crash site with Iranian authorities, Anadolu news agency said earlier on X.

State news agency IRNA said Raisi was flying in a U.S.-made Bell 212 helicopter.

Iranian Supreme Leader Ayatollah Ali Khamenei, who holds ultimate power with a final say on foreign policy and Iran's nuclear programme, sought to reassure Iranians, saying there would be no disruption to state affairs.

The chief of staff of Iran's army ordered all resources of the army and the elite Revolutionary Guards to be put to use in search and rescue operations.

Earlier, the national broadcaster had stopped all regular programming to show prayers being held for Raisi across the country. 

In the early hours of Monday, it showed a rescue team, wearing bright jackets and head torches, huddled around a GPS device as they searched a pitch-black mountainside on foot in a blizzard.

“We are thoroughly searching every inch of the general area of the crash," state media quoted a regional army commander as saying. "The area has very cold, rainy, and foggy weather conditions. The rain is gradually turning into snow."

Read also: What we know about the crashed helicopter carrying Iran's presidentSeveral countries expressed concern and offered assistance in any rescue. 

The White House said US President Joe Biden had been briefed on reports about the crash. China said it was deeply concerned. The European Union offered emergency satellite mapping technology.

The crash comes at a time of growing dissent within Iran over an array of political, social and economic crises. Iran's clerical rulers face international pressure over Tehran's disputed nuclear programme and its deepening military ties with Russia during the war in Ukraine.

Since Iran's ally Hamas attacked Israel on Oct. 7, provoking Israel's assault on Gaza, conflagrations involving Iran-aligned groups have erupted throughout the Middle East.

In Iran's dual political system, split between the clerical establishment and the government, it is Raisi's 85-year-old mentor Khamenei, supreme leader since 1989, who holds decision-making power on all major policies.

For years many have seen Raisi as a strong contender to succeed Khamenei, who has endorsed Raisi's main policies.

Raisi's victory in a closely managed election in 2021 brought all branches of power under the control of hardliners, after eight years when the presidency had been held by pragmatist Hassan Rouhani and a nuclear deal negotiated with powers including Washington.

However, Raisi's standing may have been dented by widespread protests against clerical rule and a failure to turn around Iran's economy, hamstrung by Western sanctions. 

Raisi had been at the Azerbaijani border on Sunday to inaugurate the Qiz-Qalasi Dam, a joint project. Azerbaijan's President Ilham Aliyev, who said he had bid a "friendly farewell" to Raisi earlier in the day, offered assistance in the rescue.