Militants from the Islamic State group executed 19 Yezidi girls who refused to be sex slaves by burning them alive inside iron cages, news site ARA News reported.The victims were set ablaze in the centre of Mosul on Thursday in front of a large crowd, ARA News reported, citing activists and witnesses.“They were punished for refusing to have sex with IS militants,” media activist Abdullah al-Malla was quoted as telling ARA News.“The 19 girls were burned to death while hundreds of people were watching. Nobody could do anything to save them from the brutal punishment," a witness said.IS jehadists took over 3,000 Yezidi girls as sex slaves after they overran Sinjar in northwest Iraq in August 2014, causing a mass displacement of nearly 400,000 people to Duhok and Erbil in Iraqi Kurdistan.IS continues to hold about 1,800 abducted Yezidi women and girls in Iraq and Syria, according to Kurdistan regional government officials.During a lightening offensive in 2014, the IS captured Mosul, making it the 'capital' of its self-styled Caliphate, which extends across a swathe of territory in Iraq and Syria.Supported by air strikes from the US-led coalition, by Kurdish Peshmerga troops and a Shia-dominated paramilitary force, Iraqi forces on March 24 began a long-awaited offensive to re-take Mosul from the IS.