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The deepening crisis between Iran and the US has reached a boiling point, with pro-Iran forces threatening to send US soldiers "back in coffins" should troops remain in Iraq.

Pro-Iran militias in Iraq told The Independent "all retaliation options are on the table" and they will "send American soldiers back in coffins" if the government refuses to withdraw troops from Iraq, echoing comments made by Hezbollah leader Hassan Nasrallah.

Factions within the Iraqi Popular Mobilisation Forces (PMF) have said there are "no red lines hindering their revenge" over the US airstrike on Baghdad Airport which killed Iranian general Qasem Soleimani and an Iraqi militia leader Abu Mahdi al-Muhandis, along with others.

Iran is determined to expel Americans from Iraq, and Iranian Foreign Minister Mohammad Javad Zarif told Al Jazeera that "the days of the US in this region are numbered".

"We will make the necessary deliberations and it will be an act that we will do, not in a hurry, not in a hasty manner," he said.

"However, the act by the United States had other elements - it enraged the feelings of many people outside Iran, inside Iran – that will have consequences for the United States."

Iran's Supreme Leader Ayatollah Ali Khamenei on Monday prayed and cried over the remains of Soleimani's body whilst hundreds of thousands of people took to the streets to mourn and protest American interference in the country.

Soleimani's daughter threatened violence against America, addressing Iranians earlier this week with fighting words: "The families of the American soldiers in western Asia will spend their days waiting for the death of their children."

Nasrallah vowed to target US soldiers in the region. "It is the American military who killed them and it is they who will pay the price," he said during a speech in Beirut.

He added that Hezbollah will target "US military bases, soldiers, officers and warships" and warned Donald Trump will know he lost in the Middle East when "the coffins of American soldiers and officers begin to return home".

Other US allies could also be in the firing line, Iran warned.

"Iran's revenge against America for the assassination of Soleimani will be severe… Haifa and Israeli military centers will be included in the retaliation," said former Revolutionary Guard chief Mohsen Rezaei as reported by Reuters.

Iraq threatened to expel US troops following the assassination.

Iraq's militia leader turned populist politician Muqtada Sadr reactivated his Mahdi Army on Friday, nearly a decade after dissolving the notoriously anti-US force.

Taking to Twitter, Sadr ordered "fighters, particularly those from the Mahdi Army, to be ready" following the strike, reactivating the notoriously anti-American force.

Some Iranians are protesting American interference [Getty]

Trump isn't backing down

According to a US official, the Pentagon is planning to deploy six B-52 aircrafts to the British territories of Diego Garcia in the Indian ocean on Monday, CNN reported.

The B-52s will be used against Iran if ordered, the official said.

US President Donald responded to Iran's threats with strong words of his own, and said Washington may target Iranian cultural sites, despite the fact that it is considered a war crime, according to experts.

"They're allowed to kill our people. They're allowed to torture and maim our people. They're allowed to use roadside bombs and blow up our people," he said.

"And we're not allowed to touch their cultural sites? It doesn't work that way."

There are more than 5,000 American troops still in Iraq, 17 years after the US invaded.

On Sunday, Iraq's parliament voted in favour of a non-binding resolution calling for the expulsion of the American forces.

Trump responded by saying he refuses to pull out his troops without being paid for military investments in Iraq over the years.

"We will charge them sanctions like they've never seen before," he told the press.

"It'll make Iranian sanctions look somewhat tame. If there's any hostility, that they do anything we think is inappropriate, we are going to put sanctions on Iraq, very big sanctions on Iraq," he added.

"We're not leaving until they pay us back for it."