Mohammad al-Adnani, the official spokesman of ISIS and one of its most senior members, has died in Syria, the terror group said in a rare public statement.

His death marks the highest-profile killing yet of an ISIS member. A key deputy to ISIS leader Abu Bakr al-Baghdadi, he was the person floated to be his successor should anything happen to al-Baghdadi.

A statement from ISIS' Amaq news agency on Tuesday said al-Adnani died while inspecting military operations in the area of Aleppo, Syria. ISIS has not revealed his cause of death and said it "determined to seek revenge" for the killing.

"After a journey filled with sacrifice and fight against non-believers, the Syrian Gallant knight, Abu Mohammed al-Adnani, joined the convoy of martyr leaders," ISIS said.

"To the filthy and coward nonbelievers and to the holders of the Christ emblem, we bring the good news, which will keep them awake, that a new generation in the Islamic State ... that loves death more than life ... this generation will only grow steadfast on the path to Jihad, stay determined to seek revenge and be violent toward them."

(CNN)