People who just fled the Intisar district of eastern Mosul are seen along a road in Qaraqosh, Iraq November 4, 2016. REUTERS/Zohra Bensemra

(Reuters) - Among the various Iraqi forces seeking to oust Islamic State militants from their Mosul stronghold are the Hashid Shaabi, or Popular Mobilisation Forces (PMF), a coalition of mainly Shi’ite militias who joined the U.S-backed campaign last week.

Below is some background about the forces:

- Three large militias make up the Hashid Shaabi, along with a number of smaller armed groups, totaling some 20,000 fighters. The vast majority of the fighters in the Hashid Shaabi come from Iranian-backed Arab Shi’ite militias but there are also Yazidis and Assyrians in the group. The largest of the Shi’ite militias is the Badr Organisation, headed by Hadi al-Amiri.

- Formed in 2014 to help push back Islamic State’s advance in Iraq, the PMF officially report to Prime Minister Haider al-Abadi and his Shi’ite-led government, and have led many security operations. However most of its groups have been trained by neighboring Iran.

- The fighters also have close ties with the commander of Iran’s Quds Brigade, the extra-territorial arm of the Revolutionary Guards, General Qassem Soleimani. Pictures posted on social media noted that he was recently at the frontlines around Mosul.

- The forces joined the campaign with an offensive to take the town of Tal Afar, some 55 kilometers (34 miles) west of Mosul, last Saturday, aiming to cut off any supply or retreat route for Islamic State fighters into Syria.

- Rights groups fear possible sectarian violence if the Shi’ite paramilitaries take control of areas with Sunni majorities in northern and western Iraq. Amnesty International has said Shi’ite militias in past campaigns committed “serious human rights violations, including war crimes” against civilians fleeing Islamic State-held areas. The forces and the government say abuses were isolated incidents.

- The Badr Organisation’s Amiri has said the militias were not receiving direct U.S. support in the battle. He has also said the forces could eventually head to Syria to fight Islamic State there.