ISIS confirmed on Thursday that its leader Abu Bakr al-Baghdadi was killed in a raid by US forces in northwestern Syria and it vowed revenge against the United States.
Baghdadi died during the swoop by US special forces, President Donald Trump said on Sunday.
ISIS confirmed Baghdadi's demise in an audio tape posted online and said a successor it identified only as Abu Ibrahim al-Hashemi al-Quraishi had been appointed.
The speaker in the audio also confirmed the death of Abu Hassan al-Muhajir, a close aide of al-Baghdadi and a spokesman for the group since 2016.
Al-Muhajir was killed in a joint US operation with Kurdish forces in Jarablus in northern Syria on Sunday, hours after Baghdadi blew himself up during a US raid in Syria's northwestern Idlib province.
The killings were a double blow to the extremist group, nearly seven months after its territorial defeat in Syria.
The new spokesman, al-Quraishi, addressed the Americans, saying: "Don't rejoice."
"Don't you know America that the state (ISIS) today is at the doorstep of Europe and is in Central Africa? It is also expanding and remaining from east to west."
The speaker was referencing the slogan ISIS used at the height of its successes: "Remaining and expanding."