Cairo: Islamic State confirmed on Thursday that its leader Abu Bakr al-Baghdadi was killed in a weekend raid by US special forces in northwestern Syria, and vowed revenge against the United States.

The Iraqi rose from obscurity to lead the ultra-hardline group and declare himself "caliph" of all Muslims, holding sway over huge areas of Iraq and Syria from 2014-2017 before Islsmic State's control disintegrated under US-led attacks.

The group confirmed his death in an audio tape posted online and said a successor, identified as Abu Ibrahim al-Hashemi al-Quraishi, had been appointed.

Aymenn al-Tamimi, a researcher at Swansea University focusing on Islamic State, said the name was unknown but could refer to a leading figure in Islamic State called Hajj Abdullah, whom the US State Department had identified as a possible successor.

A former senior figure in the rival Islamist group al Qaeda in Iraq, he is also known as Mohamed Said Abdelrahman al-Mawla.

Analysts have also named the Saudi Abu Abdullah al-Jizrawi and Abdullah Qaradash, an Iraqi and one of Baghdadi’s right-hand men, as potential successors along with the Tunisian Abu Othman al-Tunisi.

An Islamic State spokesman warned the United States in the tape to "beware vengeance (against) their nation and their brethren of infidels and apostates".

Baghdadi's death is likely to cause Islamic State to splinter, leaving whoever emerges as its new leader with the task of pulling the group back together as a fighting force, according to analysts.

Whether the loss of its leader will in itself affect the group's capabilities is open to debate. Even if it does face difficulties in the transition, the underlying ideology and the sectarian hatred it promoted remain attractive to many, analysts say.

Guerrilla attacks

Islamic State also confirmed the death of its spokesman Abu al-Hassan al-Muhajir.

"I think they're trying to send the message, 'Don't think you've destroyed the project just because you've killed Abu Bakr al-Baghdadi and the official spokesman'," Tamimi said.

Islamic State has resorted to guerrilla attacks since losing its last significant piece of territory in Syria in March.

Since Baghdadi's death, it has posted dozens of claims of responsibility for attacks in Iraq, Syria, Afghanistan and elsewhere.

H.A. Hellyer, senior associate fellow at the Carnegie Endowment for International peace, said the group, also known as ISIS or Daesh, would have picked the name Quraishi for Baghdadi's successor to suggest descent from the Prophet Mohammad's tribe.

Baghdadi's "caliph" name also ended in Quraishi.

"ISIS is trying to show its followers it respects that tradition, but Muslims more widely aren't likely to care very much, considering the wide violations of Islamic law that ISIS has clearly engaged with," Hellyer added.

In his last purported audio message, released in September, Baghdadi said operations were taking place daily and urged freedom for women jailed in Iraq and Syria on suspicion of links to the group.

He also said the United States and its proxies had been defeated in Iraq and Afghanistan, and that the United States had been "dragged" into Mali and Niger.