The move comes following a US strike that killed top Iranian commander Qassem Soleimani in Iraq, dramatically heightening regional tensions.
Taking to Twitter, Sadr ordered "fighters, particularly those from the Mahdi Army, to be ready" following the strike, reactivating the notoriously anti-American force.
Early Friday, a volley of missiles hit Baghdad's international airport, striking a convoy belonging to the Hashd al-Shaabi, an Iraqi paramilitary force with close ties to Iran.
Just a few hours later, Iran's Revolutionary Guard Corps announced Soleimani "was martyred in an attack by America on Baghdad airport this morning".
The Hashd confirmed both Soleimani and its deputy chief Abu Mahdi al-Muhandis were killed in what it said was a "US strike that targeted their car on the Baghdad International Airport road".
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The Pentagon said US President Donald Trump had ordered Soleimani's "killing" after a pro-Iran mob this week laid siege to the US embassy in the Iraqi capital.
The Iraqi prime minister said the strike was a "flagrant violation" of a security accord with the US, warning it would "spark a devastating war in Iraq".
A paramilitary group, Asaib Ahl al-Haq, urged its fighters to be on high alert, while in Lebanon, the leader of Iran-backed Hezbollah, Hassan Nasrallah, warned of "punishment for these criminal assassins".
The developments come after an unprecedented attack on the US mission in Baghdad.
A mob of Hashd al-Shaabi supporters surrounded the US embassy on Tuesday, angered by American airstrikes that killed 25 fighters from the network's hardline Kataeb Hezbollah faction, which is backed by Iran.
The US had acted in response to a rocket attack days earlier that had killed an American contractor working in Iraq.
Trump had blamed Iran for a spate of rocket attacks targeting US forces as well as the embassy siege, saying: "They will pay a very BIG PRICE! This is not a Warning, it is a Threat."
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