They're crucifying civilians and electrocuting prisoners en masse.

Like wounded beasts struggling to stay alive, Islamic State (ISIS) militants are lashing out with even more gruesome barbarity than before as they continue to suffer losses in the ongoing battle for the Iraqi city of Mosul.

On Sunday, an Iraqi military statement said more than 2,000 ISIS militants have been killed since Iraqi forces and U.S.-led coalition aircraft launched a major operation to liberate Mosul, Iraqi News reported.

Nineveh Operations Commander Najim Abdullah al-Jabouri said among those killed were "senior and elite leaderships in the group."

He said air cover provided by Iraqi and U.S.-led coalition air forces has already destroyed ISIS infrastructure and the majority of the group's ammunition caches.

ISIS fighters can only fight back by way of launching suicide attacks, planting of improvised explosive devices and hit-and-run raids as they hide in civilian houses, the commander said. "Our sources confirm a collective escape by group elements," al-Jabouri added.

Meanwhile, ISIS militants killed dozens of Iraqi civilians in the last two days alone on charges of passing information to "the enemy," according to local reports, the Daily Mail reported.

The militants have also electrocuted 30 prisoners who had been accused of "collaboration with security forces."

On Tuesday, ISIS militants put on display five crucified bodies at a road junction, in what is seen as an effort to frighten and dissuade the remaining 1.5 million residents of Mosul from rebelling.

Other bodies were seen hanging from electricity poles and traffic signals around the city, residents said on Wednesday.

Meanwhile, ISIS has found itself facing so tenuous a situation that its leaders have been forced to order their fighters to skip prayers and fight the advancing Iraqi joint forces instead.

The thousands of ISIS fighters in Mosul are up against a 100,000-strong coalition including Iraqi troops, security forces, Kurdish peshmerga and mainly Shi'ite paramilitary groups, which have nearly surrounded the city and broken into eastern neighbourhoods.

Sources said the militants are putting up a fiercer fight after their leader, Abu Bakr al-Baghdadi, told them in a speech last week to remain loyal to their commanders and not to retreat in the "total war" with their enemies.