BEIRUT/WASHINGTON: Daesh (ISIS) spokesman and head of external operations Abu Mohammad al-Adnani, one of the group’s longest-serving and most prominent leaders, has been killed in Aleppo province in Syria, the group said Tuesday, threatening to avenge his death.

Adnani had been one of the last remaining members alive of the group that founded Daesh, along with the group’s self-appointed caliph Abu Bakr al-Baghdadi.

As Daesh’s spokesman, he was its most visible member.

As head of external operations as well, he was in charge of attacks overseas, an increasingly important tactic for the group as its core Iraqi and Syrian territory has been eroded by military losses.

Shortly after the announcement, a U.S. defense official told Reuters the United States carried out an airstrike in Syria’s town of Al-Bab targeting a senior Daesh official, declining to disclose the target and saying the operation was still being reviewed.

A senior Syrian rebel official had said earlier that Adnani was most probably killed in Al-Bab.

Daesh’s Amaq News Agency reported that Adnani was killed “while surveying the operations to repel the military campaigns against Aleppo.” Daesh holds territory in the province of Aleppo, but not in the city, where rebels are fighting Syrian government forces.

Amaq did not say how Adnani, born Taha Subhi Falaha in Idlib province in 1977, was killed. Daesh published a eulogy dated Aug. 29 but giving no further details. Amaq vowed to revenge against the “filthy cowards in the sect of disbelief.” It said a generation raised in Daesh-held territory will take revenge.

Recent advances by the U.S.-backed Syrian Democratic Forces, an alliance of Kurdish and Arab militias, and by Syrian rebels backed by Turkey, have made inroads into Daesh holdings in Aleppo province, cutting them off from the Turkish border and supply lines along it.

There are conflicting reports as to where and how Adnani died.

A senior Syrian rebel official said Adnani was most probably killed in the Daesh-held city of Al-Bab in an airstrike. Citing unconfirmed reports, he said Adnani was in the Aleppo region to raise morale as the group comes under mounting pressure.

Hisham al-Hashimi, a security analyst who advises the Iraqi government on Daesh, said Adnani was injured in a coalition strike on Aug. 17 near Al-Rai, north of Aleppo, where Daesh is fighting Turkish and U.S.-backed Syrian rebels.

Hashimi said he died from his wounds Monday.

A U.S. counterterrorism official who monitors Daesh said that Adnani’s death will hurt the militants “in the area that increasingly concerns us as the group loses more and more of its caliphate and its financial base ... and turns to mounting and inspiring more attacks in Europe, Southeast Asia and elsewhere.”

The official said Adnani’s role as propaganda chief and director of external operations have become “indistinguishable” because the group uses its online messages to recruit fighters and provide instruction and inspiration for attacks.

Iraq said in January that Adnani had been wounded in an airstrike in the western province of Anbar and then moved to the northern city of Mosul, Daesh’s capital in Iraq.

Adnani pledged allegiance to Daesh’s predecessor Al-Qaeda more than a decade ago and was once imprisoned by U.S. forces in Iraq, according to the Brookings Institution. He has been the chief propagandist for the extremist group since he declared in a June 2014 statement that it was establishing a modern-day caliphate spanning swaths of territory it had seized in Iraq and neighboring Syria.

Adnani has often been the face of Daesh, such as when he issued a message in May urging attacks on the United States and Europe during the holy month of Ramadan.

Adnani is likely to be succeeded in his military role by the financial comptroller of the group, Iyad al-Obaidi, also known as Saleh Haifa, a former security officer for Saddam Hussein, Hashimi said.

The United States designated him a “global terrorist” this year and says he was one of the first foreign fighters to oppose U.S.-led coalition forces in Iraq since 2003 before becoming spokesman of the militant group.

There is a $5 million reward on his head under the U.S. “Rewards for Justice” program.