Small groups of guerilla fighters have allegedly begun to rise up against Islamic State militants in Mosul, which was captured by the militant group last summer, according to images from several exclusive videos cited by Sky News.

The alleged guerrillas' identities are unknown and their faces are masked as they kidnap an Islamic State (IS, formerly ISIS) official on the street in Iraq's second-largest city and force him into a car at gunpoint. The man is reportedly taken to a house where he is beaten and interrogated. The official remains defiant, however, saying: "They are stronger than you." The man is later shot dead, sources tell Sky News.



In another video the resistance movement group reportedly burns an IS house while the call to dawn prayers is heard across the city.



The third video allegedly shows an IS gunman guarding a building while looking away from the camera, which is next to a fighter with an AK-47 rifle. Shots ring out and the gunman squats to take cover. As more shots follow, he appears to be killed or badly injured.

The images are believed to be the first sign that some residents of Mosul, home to 2 million people, are hitting back, according to Sky News.



"At first they were only formed into small groups, but now they have been doing daily operations," the Kurdistan-based coordinator of the secret cells operating in Mosul told the broadcaster.



"They have become the ghosts which frighten Daesh (ISIS) and threaten Daesh in the city.



"Of course we target specific people - leaders who have influence within Daesh and their leaders inside the city. We know who is who, and who to target."



The 'ghosts' are reportedly targeting foreign fighters as a priority amid reports of tensions within the ranks of IS between outsiders and local fighters, possibly over big pay differences.



READ MORE: US, Iraq planning spring assault to retake Mosul from ISIS



The deposed governor of Mosul, Atheel Nujaifi, told Sky News that recapturing the city would "not be too difficult" because "we believe there are only about 3,000 IS fighters there." The majority of the population in the Iraqi city is Sunni Muslim.



He said many of the city's Sunnis would come out against ISIS only when the Baghdad government guarantees that Shiite militia would be kept away.



"The resistance is important because it shows others that we are prepared to get rid of IS.”



IS militants control vast territories in Iraq and Syria. Last June, the jihadists declared the captured areas a new Islamic State, a caliphate. Their leader, Abu Bakr al-Baghdadi, proclaimed himself a caliph and urged other radical Sunni groups to pledge their allegiance. When the IS attacked Mosul last year, the Iraqi army fled. US Central Command said late February that it will support an assault planned by an Iraqi and Kurdish military force of 20,000 – 25,000 troops for April or May to recapture the city of Mosul from IS fighters. If successful, it could be a strategic blow to the IS.

In June 2014, the jihadists launched a surprise attack on Camp Speicher, a former US Army base in Tikrit, where hundreds of captured Iraqi soldiers were summarily executed following a month-long siege. An Iraqi health official told Reuters earlier this month that Tikrit mass grave may contain bodies of 1,700 Iraqi soldiers killed by the IS. Shiite militias, who launched the military operation to take back Tikrit in March, have vowed to avenge the killings of the Iraqi soldiers.