Qasim was made the new commander of Hizb at a meeting of the outfit's command council chaired by Syed Salahuddin in Muzaffarabad (in Pakistan-occupied Kashmir), a spokesman of the outfit said in a statement.
Salahuddin said the high number of killings of militants in encounters were because of "traitors" and "Indian agents".
Yattoo, alias 'Gaznavi', and his two associates were killed yesterday in an encounter that began on Saturday night in Awnira village of Shopian district in south Kashmir.
Yattoo, who hailed from Budgam district of central Kashmir, had a long association with Hizbul Mujahideen and was responsible for keeping alive the prolonged unrest of 2016 following the killing of group's commander Burhan Wani in an encounter with security forces on July 8.
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He had also played key role in recruiting several young men into the outfit, according to the police.
Yattoo had joined Hizbul Mujahideen in 1996 but had surrendered in 2007 and was later released on parole in 2014. Subsequently, he had joined the militant group again and was its self-styled chief operations commander.