The Iranian government has requested the US to assist in probing the deadly helicopter crash that killed President Ebrahim Raisi, his foreign minister, and six others but Washington won't assist Tehran largely due to "logistical reasons", according to a senior American diplomat.

Raisi, 63, a possible successor for Iran's supreme leader, the 85-year-old Ayatollah Ali Khamenei, his foreign minister Hossein Amir-Abdollahian, and six others were found dead on Monday hours after their helicopter crashed in fog, leaving the Islamic Republic without two key leaders as extraordinary tensions grip the wider Middle East.

US State Department Spokesperson Matthew Miller at a press briefing on Monday said the US when asked for assistance by the Iranian government made it clear that it was ready to offer assistance, as it would do in response to any request by a foreign government in this sort of situation, but was not able to provide any help.

"I’m not going to get into the details, but we were asked by the Iranian government for assistance. We said that we would be willing to assist. It’s something that we would do with respect to any government in this situation. Ultimately, largely for logistical reasons, we weren’t able to provide that assistance," Miller told reporters.

In response to a question on the United States extending official condolences and participating in a moment of silence for Raisi at the UN Security Council, Miller said the US had been quite clear that Raisi was a "brutal participant" in the repression of the Iranian people for nearly four decades but Washington regretted any loss of life in a helicopter crash like incident.

"We regret any loss of life. We don’t want to see anyone die in a helicopter crash. But that doesn’t change the reality of his record both as a judge and as the president of Iran and the fact that he has blood on his hands," Miller said.

"Our fundamental approach to Iran has not changed and will not change. We will continue to support the people of Iran, to defend their human rights, their aspirations to an open, free society and democratic participation." Miller also responded to a remark by former Iranian foreign minister Mohammad Javad Zarif who blamed US sanctions for the incident.

Iran has offered no reasons for the crash that brought down the helicopter, which fell into mountainous terrain in a sudden, intense fog.

The crash on Sunday killed all eight people aboard a Bell 212 helicopter that Iran purchased in the early 2000s, according to the state-run IRNA news agency.

"The Iranian government has used its aircraft to transport equipment to support terrorism. So, we will continue to fully enforce our sanctions regime, including our sanctions regimes on aircraft for use by the Iranian government. Ultimately, it’s the Iranian government that is responsible for the decision to fly a 45-year-old helicopter in what was described as poor weather conditions," he said.

The Bell 212 is a two-blade, twin-engine, medium helicopter that first flew in 1968. The Bell helicopters were originally manufactured by Bell Helicopter in Fort Worth, Texas, United States before the plant was moved to Mirabel, Quebec, Canada in 1988.

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