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Iraqi authorities say they have discovered a secret factory where ISIS extremists manufactured missiles that were used to kill their enemies.

Photos broadcast by a TV station show various pieces of equipment that were recovered from the facility which was set up inside an apartment in eastern Mosul .

When they raided the flat authorities also found memos which were sent by the terror group's leadership and featured the seal of its "quality control" office, according to reports.

The factory was discovered as Iraqi troops and militias advance on Iraq's second city in a US-backed campaign to recapture ISIS last stronghold in the country.

(Image: Al-Mayadeen TV)

The missile-making factory was discovered by Nineveh provincial police in Mosul's al-Samah area, the Muslim Press reports.

Iraqi authorities have discovered several secret factories where ISIS constructed crude bombs or missiles in their fight against the extremists.

The campaign to seize Mosul is entering its fourth week as Iraqi troops and militias advance on the terror group's territory.

(Image: Al-Mayadeen TV)

Mosul has been under ISIS rule for more than two years and there are fears that many innocent civilians are being used as human shields by the terror group as it defends its shrinking territory in northern Iraq.

On Saturday Iraqi special forces backed by US-led air strikes took control of two districts during heavy street fighting in eastern Mosul.

(Image: Al-Mayadeen TV)

The military said the coalition destroyed at least 10 cars deployed by ISIS as suicide bombs and three rocket launchers.

Thirty militants, including snipers, were killed, the military said.

Lieutenant-General Abdul Wahab al-Saidi of the Counter Terrorism Service said his forces had completely taken over al-Arbajiya district and were halfway through clearing the adjacent al-Qadisiya al-Thaniya.

Last week Kurdish peshmerga fighters said they have found a network of tunnels used by ISIS in the town of Bashiqa.

The tunnels were identified by a hand-drawn map posted on a wall over an entrance.

Related video: Iraqi security forces claim to have found a mass grave near Mosul