A Syrian military source cited by Syrian state media identified the war minister as Abu Musab al-Masri, without saying exactly when or where he was killed.
He was named among 13 senior ISIS figures killed in Syrian army operations east of Aleppo, including men identified as Saudi and Iraqi nationals.
A previous ISIS minister of war, Abu Omar al-Shishani, was killed last year. The Pentagon said Shishani was likely to have been killed in a US air strike in Syria. The militant group confirmed his death in July but said he had died fighting in the Iraqi city of Shirqat south of Mosul.
ISIS faces separate campaigns in northern Syria by the Russian-backed Syrian Army, the US-backed Syrian Democratic Forces, and Turkey-backed rebels fighting under the Free Syrian Army banner.
(Writing by Tom Perry; Editing by Louise Ireland)