IRBIL, Iraq - Iraq’s state-sanctioned Shiite militias said Sunday that some 5,000 fighters have joined their push to encircle the country’s second-largest city of Mosul and cut off Islamic State of Iraq and Syria (ISIS) fighters there, as bombers killed at least 17 people in residential Shiite neighborhoods in Baghdad.

Karim al-Nuri of the umbrella group for the militias, known as the Popular Mobilization Units, and Jaafar al-Husseini, a spokesman for unit member the Hezbollah Brigades, said that a total of some 15,000 Shiite fighters were participating in the battle.

The Iraqi military confirmed the figures, which, including army units, militarized police, and special forces bring the total number of anti-ISIS fighters in the offensive to over 40,000.

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Mosul is Iraq’s second-largest city and the last major bastion of ISIS fighters in the country. The struggle to kick out the extremists has been long-anticipated since they stormed into the city in 2014, driving out a much larger Iraqi force, albeit one that was demoralized from neglect and corruption.

The involvement of the Iranian-backed Shiite militias has raised concerns that the battle for the Sunni-majority city could aggravate sectarian tensions. Rights groups have accused the militias of abuses against civilians in other Sunni areas retaken from ISIS, accusations the militia leaders deny.

The U.S. military estimates ISIS has 3,000 to 5,000 fighters inside Mosul and another 1,500-2,500 in the city’s outer defensive belt. The total number includes around 1,000 foreign fighters.

In the hours following the announcement of the Shiite reinforcements, five explosions rocked predominantly Shiite neighborhoods of the capital, killing at least 17 people and wounding over 60, police said.

Police officials said the deadliest of the bombings, a parked car bomb, hit a popular fruit and vegetable market near a school in the northwestern Hurriyah area, killing at least 10 and wounding 34. Other attacks by improvised explosive devices hit the northern Shaab neighborhood, as well as traders’ markets in the Topchi and Zataria areas as well as the poorer Sadr City district.

The officials spoke on condition of anonymity because they weren’t authorized to brief reporters.

There was no immediate claim of responsibility. But ISIS has stepped up its attacks in response to the offensive in Mosul, and it was possible the group was targeting Shiite areas in retaliation for the Mosul offensive.

Earlier, Turkey’s president warned that his government will be closely monitoring the Shiite militias’ behavior in northern Iraq and seek to safeguard the rights of ethnic Turkmens there.

In statements carried by state-run Anadolu agency, Recep Tayyip Erdogan told reporters that the militia group, known in Arabic as Hashd al-Shaabi, could prompt a Turkish response if it “terrorizes” the Iraqi-Turkmen town of Tel Afar, where it is headed in its push around Mosul.

“Tel Afar is an entirely Turkmen town. If Hashd al-Shaabi starts terrorizing it, then our response will certainly be different,” Erdogan said.

The use of government-backed Shiite militias is a sensitive issue in the ongoing battle to retake Mosul from ISIS. Militia commanders have said their forces will not enter Mosul but will instead focus on surrounding towns like Tel Afar.

Also Sunday, military officials reported that an army helicopter had crashed in the central province of Salahuddin due to a technical malfunction. They said that the two pilots of the helicopter, which went down the previous evening, were missing. They spoke on condition of anonymity because they weren’t authorized to brief reporters.