The Islamic State (Isis) has begun to lash out with greater brutality and frequency on the citizens and fighters in and around its Iraqi capital Mosul, as Iraqi and Kurdish forces backed by US air strikes close the noose around the city.

The extremist Islamist group is quicker to kill deserters and is relying on recruits, some as young as 13, as the forces arrayed against them prepare for a final push into Mosul. For months, Iraqi troops from the south and Kurdish forces from the north have been clawing back towns surrounding the city.

One refugee from the town of Qayara to the south of Mosul, which remains in IS hands, described to the Associated Press how fighters with the militant group were becoming increasingly nervous as they heard about defeats across the country.

Jarjis Muhammad Hajaj, who was among thousands of residents who fled fighting in the area and now live in the Dibaga Camp, said beheadings and extrajudicial killings had become commonplace as Isis becomes increasingly paranoid about spies and hunts down deserters.

"They started making raids on houses, arresting people and beheading them," he explained.

Hajaj also commented that the group appeared to be collapsing as they boosted their ranks with younger fighters. Isis militants as young as 13 or 14 have been patrolling the streets in Awsaja, over the Euphrates from Qayara, according to one resident who spoke on condition of anonymity. He added that Isis wanted those civilians still in the town to remain as human shields.

Mosul and the area surrounding it has been under IS rule for two years, ever since the Sunni militants overran much of western and northern Iraq, joining it to the territory they control in neighboring Syria in a self-declared "caliphate."

US and Iraqi officials say the final assault on Mosul is still weeks away as forces fight to retake territory around the city. From the Qayara military base, Iraqi troops are still 70 kilometers (40 miles) from the city.