The main spokesman for the Islamic State of Iraq and the Levant (ISIL) armed group, Abu Mohamed al-Adnani, has been killed in the Syrian province of Aleppo, according to an ISIL-linked website.

Amaq, the ISIL-affiliated media, said on Tuesday Adnani was killed while monitoring military operations in Aleppo.

A US defence official said the US-led coalition forces battling ISIL had conducted an air strike Tuesday targeting a "senior leader" of the group, without specifying who the leader was.

"Coalition forces conducted an air strike in al-Bab, Syria, targeting an [ISIL] senior leader," the official said, according to the Paris-based AFP news agency.

"We are still assessing the results of the operation at this time."

Adnani, described as ISIL's second most senior leader, was one of the group's longest-serving figures.

The details of Adnani's death remain unclear.

Al Jazeera's Patty Culhane, reporting from Washington DC, said there was no immediate comment or confirmation from US officials of his death.

Believed to had been born in Syria, Adnani "was often seen on video urging followers to carry out attacks abroad", our correspondent said.

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Adnani has been the voice of ISIL, also known as ISIS, over the past few years, and has released numerous, lengthy audio files online in which he urged followers to carry out attacks.

"Adnani commanded widespread respect within the movement [ISIL]," Shiraz Maher, Deputy Director of the International Centre for the Study of Radicalisation at King’s College London, said.

"What made him particularly dangerous was that he personally oversaw and directed ISIL's external operations. His prominence meant that he even eclipsed the leader of ISIL, Abu Bakr al-Baghdadi, in terms of public pronouncements, so the group will feel his absence quite pointedly."

Earlier this year, Adnani called for massive attacks during the Muslim holy month of Ramadan. He has also called for attacks in Western countries.

ISIL controls large swaths of territory in Syria and Iraq, and is fighting Syrian troops, US-backed fighters and other rebel groups in northern Syria, as well as in Iraq.