Photos: Iraqi troops face Isis snipers and suicide car bombs in fierce battle for western Mosul

Iraqi troops have faced their heaviest clashes in their battle to reclaim western Mosul from Isis fighters. The militants dispatched at least six suicide car bombs, which were all destroyed before reaching the troops. A senior military source said jihadist mortar teams and snipers are moving from house to house.

The wave of heavy resistance comes as Iraqi forces launched attacks against Isis-held neighbourhoods in western Mosul from three points. The Federal Police are closing in on the city's main government complex in the Dawasa area, while Iraq's special forces are attempting to push into the Shuhada and Mansour districts.

Iraqi forces have captured Mosul's al-Hurriya bridge, which leads to the Isis-held old city from the south, a military media officer told Reuters. This is the second of the city's bridges to be secured by Iraqi forces, after securing one located further south. All of Mosul's five bridges over the Tigris were destroyed, and their capture and repair should help the offensive against the militants.

Iraqi forces have discovered an underground Isis training camp in a tunnel in a hillside overlooking Mosul. In less turbulent times, trains ran through it on their way to or from the city, but when the militants overran the area in the summer of 2014, they barricaded both ends, ripped up the tracks and built an assault course inside, on which to train their recruits.

The railway was built in the early 20th century, as part of the line connecting Berlin to Baghdad. Isis slogans are painted on the walls of the tunnel. "By the will of God, we will conquer Rome," reads one mural painted on the wall of the tunnel against the background of a blood red sun.

More than 40,000 people have fled Mosul in the past week. The pace of displacement has accelerated in recent days as fighting approaches the most densely populated parts of western Mosul, and aid agencies have expressed concern that camps to accommodate people fleeing the city are almost full.

The International Organisation for Migration's Mosul Displacement Tracking Matrix showed the number of people uprooted since the start of the offensive in October exceeded 206,000 on Sunday (5 March), up from 164,000 on 26 February. That number may still rise sharply. The United Nations last month warned that more than 400,00 people, more than half the remaining population in western Mosul, could be displaced.

The Organisation for the Prohibition of Chemical Weapons said in a statement it was "seriously concerned" about reports of chemical weapons use in Mosul. The alleged attack occurred in eastern Mosul, an area declared fully liberated by Iraqi forces in January. Hospital officials said 10 patients were admitted for exposure and would be discharged in the coming days.

The United Nations warned that the use of chemical weapons, if confirmed, would be a war crime and a serious violation of international humanitarian law.

The push on Mosul's west was launched on 19 February; the eastern half of the city was declared "fully liberated" in January. The operation to retake Mosul officially began in October after more than two years of slowly clawing back territory from Isis militants.