U.S. President Donald Trump warned Sunday Iran’s next supreme leader would not last long, as Tehran prepared to announce the successor to the slain Ayatollah Ali Khamenei.

Nine days after U.S.-Israeli strikes on his compound killed Khamenei and plunged the Middle East into war, Iran's Assembly of Experts met privately and chose their next leader, members of the body said.

The clerics did not say who had been selected, only that a name would be announced soon. Some suggested Khamenei's 56-year-old son Mojtaba Khamenei would succeed his father.

Trump had previously demanded a say in the appointment and dismissed the younger Khamenei as an unacceptable "lightweight".

"He's going to have to get approval from us," Trump told ABC News on Sunday, referring to Iran's next leader. "If he doesn't get approval from us he's not going to last long."