Iran's Islamic republic leaders have named hard-liner Mojtaba Khamenei to replace his father, the late Ayatollah Ali Khamenei, as supreme leader in a decision that comes after President Donald Trump warned anyone chosen won't "last long" without US approval.

Iran's Assembly of Experts, responsible for electing the new ruler, said on March 8 that it has "designated and introduced Ayatollah Seyyed Mojtaba Hosseini Khamenei as the third Supreme Leader of the sacred system of the Islamic Republic of Iran."

As supreme leader, Mojtaba Khamenei will have final say in all matters of state and hold near-dictatorial matters.

Ayatollah Ali Khamenei was killed in a US-Israeli air strike on February 28, the first day of the war that has spread throughout the region over the past week.

Trump had earlier said that Mojtaba Khamenei would be an "unacceptable choice."

The US president, who had previously called on Tehran to stop its resistance and surrender, had also told the American TV network ABC that without US approval, the new Iranian leader "is not going to last long."

In announcing the decision, Iran's Assembly of Experts appeared defiant, saying it made the choice "despite the acute war situation and direct threats from enemies."

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The United States and Israel carried out new air strikes on strategic infrastructure in and around Tehran on March 8, including oil depots and refining facilities, as Israeli forces also expanded their operations beyond Iran.

Large fires were reported at several fuel storage sites in Tehran, sending heavy smoke over parts of the capital. Iranian officials confirmed that multiple oil facilities were damaged, although emergency teams managed to bring some fires under control.