The thousands who did gather in the centre of Iran's capital were instead mourning Khamenei's death, according to AFP journalists.

The mourners, dressed mostly in black and some crying, chanted "death to America" and "death to Israel" in Enghelab (Revolution) Square, with many waving Iran's flags and holding photos of Khamenei.

Iran's President Masoud Pezeshkian on Sunday branded Khamenei's killing a "declaration of war against Muslims" by Israel and the United States.

Iran's powerful Revolutionary Guards vowed earlier on Sunday to punish Khamenei's "murderers".

Cheers and mourning in Iran. Euphoric celebrations against the Iranian regime and angry protests against the war. Uncertainty of what comes next.

The world greeted with jubilation, anger or trepidation the news Sunday that Iran's supreme leader Ayatollah Ali Khamenei had been killed in the US-Israeli strikes on the Islamic republic a day earlier.

Iranians took to the streets cheering with joy, setting off fireworks and playing celebratory music after reports of Khamenei's death, according to witnesses and video footage verified by AFP.

The celebrations in Tehran began shortly after 11pm (8.30am NZT), according to multiple witnesses and audio recordings.

People were not, however, coming out en masse to celebrate, according to social media.

Many Iranians were fearful after the deadly crackdown on mass anti-government protests in January.

Three members of the US military have been killed and five others wounded in the operation against Iran, the Pentagon says, announcing the first American deaths in the US-Israeli campaign that killed the Islamic republic's supreme leader.

Iran launched a new round of retaliatory attacks across the Gulf after vowing to avenge the slain Ayatollah Ali Khamenei, defying a threat from President Donald Trump to strike with unprecedented force.

As crowds gathered in Tehran, with some grieving and others celebrating, explosions rang out and the Israeli military announced it was again striking targets in the heart of the capital.

"Three US service members have been killed in action and five are seriously wounded as part of Operation Epic Fury," the Pentagon said, adding several others suffered minor shrapnel injuries and concussions. "Major combat operations continue and our response effort is ongoing."

President Donald Trump says 48 Iranian leaders have been killed in the US-Israeli bombardments of the country and that the offensive is "very positive".

"Nobody can believe the success we're having, 48 leaders are gone in one shot. And it's moving along rapidly," Trump was quoted as saying in an interview by Fox News.

Trump claimed overall success in the war, which was launched Saturday with the goal of removing the Islamic republic's leadership and destroying its military. Iran has also confirmed the death of its supreme leader, Ayatollah Ali Khamenei.

"We're doing our job not just for us but for the world. And everything is ahead of schedule," Trump was quoted as saying in a separate interview with CNBC.

"Things are evolving in a very positive way right now, a very positive way," he said.

The interviews were conducted before the US military for the first time announced casualties in the war: three unidentified service members killed, five seriously wounded and several others more lightly injured.

Central Command (Centcom) also announced that the United States had sunk an Iranian warship at a dock in the Gulf of Oman.

Pro-Iranian protesters angered by the death of Iranian Supreme Leader Ayatollah Ali Khamenei tried to storm the US consulate in Pakistan's Karachi on Sunday, leaving eight dead, and the fortified Green Zone hosting Washington's embassy in the Iraqi capital Baghdad.

Iranian state media confirmed the death of Khamenei, Iran's supreme leader since 1989 and sworn enemy of the West, on Sunday, after the opening salvo of a massive US and Israeli attack.

In the Pakistani megacity of Karachi, hundreds of pro-Iranian protesters tried to enter the US mission, an AFP journalist saw.

At least eight people were killed in the protests and at least 20 were injured, Muhammad Amin, a spokesman for the Edhi Foundation rescue service said, adding most had bullet wounds.

A crowd of young people climbed over the main gate and gained access to the driveway of the consular building, smashing some windows.

Police fired tear gas at the protesters who dispersed.

Videos on social media showed youngsters smashing the windows of the main building of the consulate as the American flag could be seen flying over the compound, whose perimeter is topped with barbed wire.

Thousands of people were also taking to the streets in the eastern city of Lahore and in northern Skardu, with a demonstration expected in the afternoon near the diplomatic enclave housing the US embassy in the capital Islamabad.

Khamenei death 'hurt us'

In Baghdad, hundreds of Iraqis, many dressed in black, attempted to storm the compound housing the American embassy on Sunday despite a heavy security deployment.

Ali, a masked protester, told AFP "the martyrdom of Sayyed Ali Khamenei has hurt us".

"We are here because we want the withdrawal of the occupying American forces from Iraq," he said, referring to US-led coalition troops who have recently reduced their presence and are now mostly stationed in northern Iraq.

Protesters, who had gathered in the Iraqi capital since early Sunday, hurled stones at security forces, who responded with tear gas.

A security source told AFP earlier that "their attempts have been thwarted so far, but they keep trying".

In Indian-administered Kashmir, several thousand Shia Muslims joined street demonstrations in the main city Srinagar.

Protesters holding red, black, and yellow flags converged on the main square.

Many of them chanted anti-Israel and anti-US slogans during the emotionally charged but largely peaceful gathering.

"This day we are all very heavy-hearted. We are mourning our beloved leader who was martyred," Syed Towfeeq, 40, told AFP.

"We all have a message for [US President Donald] Trump ... We will always stand against your oppression."

Similar protests were held in other places across Kashmir and other parts of India with a sizeable Shia Muslim presence.

- AFP

Iran's army said it targeted on Sunday US bases in Iraq's Kurdistan region and in the Gulf in response to Israeli and US strikes that killed supreme leader Ayatollah Ali Khamenei

"A few minutes ago, pilots of the air forces of the Islamic Republic of Iran successfully bombed US bases in the countries of the Persian Gulf and in the Kurdistan region of Iraq over several phases of operations," Iran's army said in a statement carried by state TV.

- AFP

Iran's armed forces chief of staff Abdolrahim Mousavi was killed along with other senior generals in US and Israeli strikes on the country, state TV reported on Sunday.

State TV listed the name of Mousavi along with defence minister Aziz Nasirzadeh, Revolutionary Guards chief Mohammad Pakpour, and head of the defence council Ali Shamkhani as among those killed during Saturday's strikes.

It said they were killed "during a Defence Council meeting", adding that other names will be announced later.

- AFP

Iran's security chief Ali Larijani said the transition process following the killing of supreme leader Ayatollah Ali Khamenei will begin from Sunday.

"An interim leadership council will soon be formed. The president, the head of the judiciary and a jurist from the Guardian Council will assume responsibility until the election of the next leader," said Larijani, the head of Iran's top security body, the Supreme National Security Council, who was an adviser to Khamenei.

"This council will be established as soon as possible. We are working to form it as early as today," he said in an interview broadcast by state TV.

Larijani warned on Sunday against attempts to divide Iran following the killing of supreme leader Ayatollah Ali Khamenei in US and Israeli strikes.

"Groups seeking to divide Iran should know that we will not tolerate it," Larijani said in an interview broadcast on state TV, calling on Iranians to unite.

- AFP

US President Donald Trump said on Sunday the United States would hit Iran with "force that has never been seen before" if the country retaliated against the US and Israeli strikes that killed supreme leader Ayatollah Ali Khamenei.

"Iran just stated that they are going to hit very hard today, harder than they have ever hit before," Trump wrote on his Truth Social platform.

"THEY BETTER NOT DO THAT, HOWEVER, BECAUSE IF THEY DO, WE WILL HIT THEM WITH A FORCE THAT HAS NEVER BEEN SEEN BEFORE!"