Just before midnight, AFP correspondents in Baghdad heard two loud blasts followed by the wailing security sirens of the Green Zone.

The attack came nearly 24 hours after Tehran launched ballistic missiles at Iraqi bases housing American and other coalition forces, which did not cause casualties.

The strikes were in retaliation for a US drone strike that killed top Iranian general Qasem Soleimani and Iraqi commander Abu Mahdi al-Muhandis last week.

Muhandis had been the deputy head of the Hashed al-Shaabi, a web of armed groups incorporated into the Iraqi state but which also have close ties to Tehran.

The United States had accused Hashed groups of being behind a string of rocket attacks on the US embassy in Baghdad and bases hosting American troops across the country.

On Wednesday, the Hashed's hardline factions vowed they, too, would take revenge for the US raid.

Paramilitary chief Qais al-Khazali -- blacklisted as a "terrorist" by the US -- said Iraq's response to the US "will be no less than the size of the Iranian response."

Harakat al-Nujaba, a hardline Hashed faction, vowed to avenge Muhandis.

"To American soldiers: Do not close your eyes. Revenge for the martyr Muhandis is coming at the hands of Iraqis -- until the last soldier among you leaves," it said.