Countries around the world voiced fears of a Middle East conflagration as top Iranian officials vowed on Sunday to avenge their slain supreme leader.

United Nations chief Antonio Guterres condemned the strikes. "I call for an immediate cessation of hostilities and de-escalation," he said in a statement, adding that the attacks on both sides undermined international peace and security.

UN rights chief Volker Turk said further attacks would "only result in death, destruction and human misery".

On Sunday, Iranian top officials vowed to avenge their slain Supreme Leader Ayatollah Ali Khamenei and declared a new wave of strikes on US bases in the Gulf.

A day before, Israel and the United States launched joint attacks on Tehran and several other Iranian cities, including Tabriz, Qom, Isfahan, Kermanshah, and Karaj. Iran responded with missile and drone strikes targeting Israel and US bases across the region.

Russia condemned the US and Israeli strikes on Iran, warning they were "bringing the region to the brink of a humanitarian, economic, and — this cannot be ruled out — radiological catastrophe".

The governments of France, Germany and the United Kingdom said they did not participate in the strikes and urged Tehran to end its nuclear program and seek "a negotiated solution".

In a joint statement, President of the European Commission Ursula von der Leyen and President of the European Council Antonio Costa said the situation in Iran is "greatly concerning", and reaffirmed their "steadfast commitment to safeguarding regional security and stability".

Canada's Prime Minister Mark Carney and Foreign Minister Anita Anand said in a statement that Ottawa was closely following the situation and urged Canadians in Iran to shelter in place.

Across the Middle East, governments and bodies voiced fears that the confrontation could spiral into a broader regional war.

Oman, which has mediated recent US-Iran talks, expressed dismay that negotiations had been undermined and urged an immediate halt to military operations.

The Arab League appealed to all international parties "to work toward de-escalation as soon as possible, to spare the region the scourge of instability and violence, and to return to dialogue".

Lebanese militant group Hezbollah has vowed to confront the US and Israel over their strikes on Iran.

On Sunday, Israel and Iran traded strikes, The Associated Press reported.

Blasts in Tehran — whose target was not immediately clear — sent a huge plume of smoke into the sky in an area where there are government buildings. Iranian authorities say more than 200 people have been killed since the start of the US and Israeli strikes.

Loud explosions caused by missile impacts or interceptions could be heard in Tel Aviv. Israel's rescue services said eight people were killed and 28 wounded in a strike in the central town of Beit Shemesh, bringing the overall death toll in the country to 10.

Meanwhile, Iran's Expediency Discernment Council has selected Alireza Arafi as a member of the country's interim leadership council, the official news agency IRNA reported on Sunday.

Earlier, Iran announced the formation of a three-member transitional council to handle the state duties after Khamenei was killed.

The interim leadership council also comprises Iran's President Masoud Pezeshkian and Judiciary Chief Gholamhossein Mohseni-Ejei.

Pezeshkian said the new leadership council "has begun its work".

Agencies, Xinhua and Yang Gao in Toronto contributed to this story.