The strike targeting Soleimani early on Friday also killed Abu Mahdi al-Muhandis, deputy commander of Iraq's Hashd al-Shaabi militias, or Popular Mobilisation Forces.
"All fighters should be on high alert for upcoming battle and great victory," said Khazali, chief of the Iran-backed Asaib Ahl al-Haq (League of the Righteous) militia, according to Reuters.
"The price for the blood for the martyred commander Abu Mahdi al-Muhandis is the complete end to American military presence in Iraq," the militia leader said.
Khazali's Asaib Ahl al-Haq is one of the founding militias part of Muhandis' Popular Mobilisation Forces.
Alongside Soleimani, widely known as the architect of Iran's regional security apparatus, five others were killed in the US air raid carried out on the directions of President Donald Trump, early reports suggested.
Tehran has threatened swift retaliation against Washington for the shock strike, with Supreme Leader Ayatollah Ali Khamenei promising "severe revenge".
"The US bears responsibility for all consequences of its rogue adventurism," Iranian Foreign Minister Javad Zarif said in a tweet, terming the killing a "foolish escalation".
President Hassan Rouhani added: "There is no doubt that the great nation of Iran and the other free nations of the region will take revenge for this gruesome crime from criminal America."
The attack came amid tensions with Washington after a New Year's Eve attack by Iran-backed militias on the US Embassy in Baghdad, prompting Trump to order about 750 US soldiers deployed to the Middle East.
The two-day embassy siege, which ended Wednesday, was triggered by US airstrikes on Sunday that killed 25 fighters from Kataib Hezbollah, another Iran-backed Iraqi militia and key component of Hashd al-Shaabi. The deadly strikes came in retaliation against the killing of an American contractor in a rocket attack on an Iraqi military base last week.
Prior to the strike that killed Soleimani and Muhandis, US officials had suggested they were readying to engage in further retaliatory strikes on Iraq.
"The game has changed," Defence Secretary Mark Esper said Thursday, telling reporters that violent acts by Iran-backed Shia militias in Iraq will be met with US military force.
"We are prepared to exercise self-defence, and we are prepared to deter further bad behaviour from these groups, all of which are sponsored, directed and resourced by Iran."
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