The Islamic State group says its spokesman has been "martyred" in northern Syria.

The IS-run Aamaq news agency said Tuesday that Abu Muhammed al-Adnani was "martyred while surveying the operations to repel the military campaigns against Aleppo," without providing further details.

IS has released several audio files online in which Adnani, a senior leader in the group, delivers fiery sermons urging followers to carry out attacks.

The extremist group has suffered a string of defeats in recent weeks, including in Syria's northern Aleppo province, where Turkish troops and allied Syrian rebels drove IS out of the border town of Jarablus last week.