Iran's foreign minister has challenged United States President Donald Trump in a tweet, sharing photos from the massive crowds in Tehran mourning an Iranian commander killed by a recent US drone strike.

Key points: Esmail Ghaani has been appointed as the new leader of the elite Quds Force

Esmail Ghaani has been appointed as the new leader of the elite Quds Force Iran's Supreme Leader called him "one of the most prominent commanders"

Iran's Supreme Leader called him "one of the most prominent commanders" However, very little is known about Mr Ghaani

Mohammad Javad Zarif wrote, "Have you EVER seen such a sea of humanity in your life, @realdonaldtrump?" and urged Mr Trump to distance himself from his advisers who seek confrontation with Iran.

"Do you still want to listen to the clowns advising you on our region? And do you still imagine you can break the will of this great nation?", the tweet read.

A satellite image shows crowds stretching nearly 6 kilometres in central Tehran. ( Supplied: Maxar Technologies )

The huge processions for Major General Qassem Soleimani mark the first time Iran has honoured a single man with a multi-city ceremony.

Police said attendees entered into the millions.

Analysts have described the death of General Soleimani as a watershed moment, even in the long and bloody history of Middle East conflict.

General Soleimani was the head of the elite Quds Force of the Islamic Revolutionary Guard Corps and had long been seen by Israel and the US as one of the most dangerous and potent figures in the region.

In a rare display of emotion from the typically reserved and measured Supreme Leader of Iran, Ayatollah Ali Khamenei cried openly at the funeral.

Mr Khamenei's voice cracked under the weight of the moment during a funeral procession unlike any in Iran's recent history.

The Quds Force is part of the 125,000-strong Revolutionary Guard, a paramilitary organisation that answers only to Iran's supreme leader.

The Revolutionary Guard has seen its influence grow ever stronger, both militarily and politically, in recent decades.

Iran's Quds Force has played a role in stoking regional conflicts, with Tehran bolstering its military clout abroad through proxies across the region. It is believed to have supplied support and weapons to Bashar al-Assad's regime in Syria, as well as to Houthi rebels in Yemen and Hezbollah in Lebanon.

Quds force 'will be unchanged': Khamenei

The newly-appointed leader of Iran's Quds Force, Brigadier General Esmail Ghaani, joined the country's leaders at the funeral of General Soleimani.

General Esmail Ghaani has been appointed as the head of Iran's Quds Force. ( AP: Hossein Zohrevand via Tasnim News Agency )

One of his first duties will likely be to oversee whatever retaliation Iran intends to seek for the US airstrike which killed his longtime friend.

General Ghaani will also be responsible for marshalling Tehran's proxies across the Middle East as the Islamic Republic threatens the US with "harsh revenge".

Mr Khamenei called the new leader "one of the most prominent commanders" in service to Iran, during the announcement of his appointment.

The Quds Force "will be unchanged from the time of his predecessor," Mr Khamenei said, according to state news agency IRNA.

General Ghaani has only occasionally come up in both Western or Iranian media, but his personal story broadly mirrors that of General Soleimani.

Born on August 8, 1957 in the north-eastern Iranian city of Mashhad, General Ghaani grew up during the last decade of Iran's monarchy.

He joined the Guard a year after the 1979 revolution and like General Soleimani, he was first deployed to put down the Kurdish uprising in Iran that followed the Shah's downfall.

He joined the Quds Force shortly after its creation and worked with General Soleimani, as well as leading counterintelligence efforts at the Guard.

"We are children of war," he once said of his relationship with General Soleimani, according to IRNA.

"We are comrades on the battlefield and we have become friends in battle."

'He'll have little difficulty filling Soleimani's shoes'

The Revolutionary Guard is one of the most powerful institutions in Iran.

Western analysts believe General Soleimani focused on the nations west of Iran, while General Ghaani's remit was those to the east like Afghanistan and Pakistan

However, this remains speculation, as Iranian state media has not elaborated on his time in the Guard.

Afshon Ostovar, the author of a book on the Guard, said the fact that General Ghaani had survived at such high ranks in the Guard, and remained General Soleimani's deputy for so long, "says a lot about the trust both Khamenei and Soleimani had in him."

"I suspect he'll have little difficulty filling Soleimani's shoes when it comes to operations and strategy."

In 2012, the US Treasury placed sanctions on General Ghaani, describing him as having authority over "financial disbursements" to proxies affiliated with the Quds Force.

The sanctions particularly tied him to an intercepted shipment of weapons seized at a port in 2010 in Nigeria's most-populous city, Lagos.

Also in 2012, General Ghaani drew criticism from the US State Department after reportedly saying, "If the Islamic Republic was not present in Syria, the massacre of people would have happened on a much larger scale."

In January 2015, he indirectly said Iran had sent missiles and weapons to Palestinians to fight Israel.

'Iran must refrain from further violence': NATO

People march as they take part in an anti-war protest amid increased tensions between the United States and Iran at Times Square in New York, U.S., January 4, 2020. ( Reuters: Eduardo Munoz )

As tempers flared in Tehran over General Soleimani's killing, NATO called for a de-escalation of tensions.

"We are united in condemning Iran's support of a variety of different terrorist groups," NATO Secretary-General Jens Stoltenberg said after an emergency meeting to discuss the rising tension in the region.

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"At the meeting today, allies called for restraint and de-escalation.

"A new conflict would be in no-one's interest, so Iran must refrain from further violence and provocations."

Mr Stoltenberg also said the training of troops by NATO forces in Iraq would be temporarily suspended.

"We are taking all precautions necessary to protect our people," he said, before adding the training could be restarted "when the situation permits".

On Sunday, Prime Minister Scott Morrison said no decision had been made to pull Australian troops from Iraq.

There are around 300 Australian Defence Force members and diplomatic personal currently in Iraq, and the Australian embassy in Baghdad remains on lockdown, with all non-essential staff ordered to leave the country.

AP/ABC