The Islamic State's propaganda chief and head of 'external operations' has been killed during "operations" in Syria's Aleppo, the group said on Tuesday.

"Shaykh Abu Muhammad al-Adnani, the spokesman of the Islamic State, was martyred while surveying the operations to repel the military campaign against Aleppo," the group said through its propaganda arm, Amaq.

Adnani was reportedly a senior figure in the group's Emni section, responsible for planning and carrying out IS's international attacks, and was considered by some to be the second-in-command after current IS head Abu Bakr al-Baghdadi.

Amaq did not give details of how and when Adnani was reportedly killed.

A source in the northern Syrian city told local news site Aleppo 24 that Adnani was likely one of those killed when US-led, anti-IS coalition forces targeted a car carrying alleged IS operatives close to the small town of al-Bab, north-east of Aleppo city.

The United States said coalition forces had carried out an air strike targeting Adnani in Aleppo province on Tuesday and that it was still assessing the results of the raid, but that his death would be a major blow to the group.

In Washington, Pentagon spokesman Peter Cook said Adnani "has served as principal architect of ISIL's external operations and as ISIL's chief spokesman," using another name for the group.

"He has coordinated the movement of ISIL fighters, directly encouraged lone-wolf attacks on civilians and members of the military and actively recruited new ISIL members," he said.

Adnani, who had a $5 million US bounty on his head, was originally from the western Syrian province of Idlib and joined the militant movement in Iraq, where he served under late al-Qaeda leader Abu Musab al-Zarqawi.

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

Recent advances by US and Turkey-backed Kurdish and Arab militias have made inroads into Islamic State territory in Aleppo province, cutting them off from the Turkish border and supply lines along it.

Iraq said in January that Adnani had been wounded in an air strike in the western province of Anbar and was then moved to the northern city of Mosul, Islamic State’s capital in Iraq.

Adnani was a Syrian from Idlib, southwest of Aleppo, who pledged allegiance to Islamic State's predecessor al-Qaeda more than a decade ago. He was also once imprisoned by US forces in Iraq, according to the Brookings Institution.

He has been the chief propagandist for the group since he declared in a June 2014 statement that it was establishing a modern-day caliphate spanning large swathes of territory it had seized in Iraq and neighbouring Syria.

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

A former IS operative from Germany, currently in prison on terrorism charges, told the New York Times earlier this month how Adnani had co-ordinated monthly meetings to select propaganda material to promote the group's battlefield advances, as well as masterminding international attacks.

"The big man behind everything is Abu Muhammad al-Adnani," the former IS member, Harry Sarfo, told the US daily.