Four Iraqi militia brigades have reportedly split from the Popular Mobilisation Forces and are now under the directives of the office of the departing Prime Minister Adel Abdul Mahdi.

The killing of the PMF's chief of staff Abu Mahdi Al Muhandis in January by the US deprived the forces of their leading policymaker and challenged the standing of Iran-backed militias in Iraq.

In a letter addressed to Faleh Al Fayyad, the new head of the PMF, also known as Hashed Al Shaabi, Mr Abdul Mahdi said “their operations and administration” will now be directed by his office.

“The details will be disclosed at a later time,” the letter said.

The brigades who have split are the Imam Ali, Ali Al Akbar, Abbas and Ansar Al Marjaiya, all loyal to the top Shiite cleric Ayatollah Ali Al Sistani.

Mr Abdel Mahdi is no longer in office, with former head of intelligence Mustafa Al Kadhimi having taken up the role as Prime Minister designate this month.

He is still considered the last holder of the office, however, until a new leader can be decided on by Parliament.

The departing prime minister has come under pressure from Washington to curb Iran's influence in Iraq, particularly after recent unclaimed attacks against US interests in the country.

The militias, which helped Iraqi and US-led international coalition forces to drive out ISIS, have broad influence in Iraqi politics.

But Iraq declared victory against the militants in 2017. The decision is a direct message to the PMF that Mr Al Sistani's religious call to form a coherent fighting force against ISIS has come to an end, an Iraqi parliamentarian told The National.

“The Marjaiya [brigade] and the outgoing prime minister made this decision. That in itself sends a strong message,” he said.

An electoral alliance made up of militia leaders and fighters came second in a 2018 parliamentary election and went on alongside populist cleric Moqtada Al Sadr, whose political group came first, to nominate Mr Abdul Mahdi as premier.

Mr Abdul Mahdi issued a decree ordering the militias to come to heel by the end of July 2019, or be deemed “illegitimate”.

His decree forces groups that make up the PMF to choose between political and paramilitary activity.