Israel's military has threatened to target any successor to former supreme leader Ali Khamenei, who was killed in the first wave of US and Israeli strikes on Iran.

US President Donald Trump had previously dismissed Mojtaba Khamenei as a "lightweight", and insisted he should have a say in appointing a new Iranian leader.

China's foreign ministry spokesman Guo Jiakun told reporters on Monday that Iran's decision to appoint the younger Khamenei was "based on its constitution".

"China opposes interference in other countries' internal affairs under any pretext, and Iran's sovereignty, security, and territorial integrity should be respected," he said when asked about the threats against the new leader.

Israel and the United States have continued pounding Iran in recent days, with Tehran retaliating by launching waves of missile and drone attacks on Israel and Gulf neighbours hosting US forces.

Beijing is a close partner of Tehran and condemned the killing of the former supreme leader, but it has also criticised the Iranian strikes against Gulf states.

China's envoy to the Middle East urged de-escalation when he met Saudi Arabia's foreign minister for talks on Sunday.

"China urges all parties to immediately cease military operations, prevent further escalation of tensions, and avoid causing greater harm to the people of regional countries," Zhai Jun told his counterpart, Faisal bin Farhan.

China's Foreign Minister, Wang Yi, said Sunday that the war "should never have happened" and called for an end to fighting.

The conflict has threatened global energy security and trade, as well as China's oil supplies.

More than 80 percent of Iranian oil exports went to China last year, according to analytics firm Kpler.

That accounted for a relatively small proportion of China's total seaborne oil imports, standing at about 13 percent, Kpler data shows.

However, more than half of China's total seaborne crude imports last year came from the wider Middle East, Kpler said, making it highly dependent on transit through the Strait of Hormuz.