The ISIS leader at the top of the U.S. kill list has died after being wounded in Syria, a U.S. military intelligence official told NBC News Tuesday.

Abu Muhammad al-Adnani was the terror group's No. 2 commander, its charismatic director of external operations and main propagandist.

He succumbed to injuries sustained in Al-Bab, a town 15 miles northeast of Aleppo and 20 miles from the Turkish border, the official said.

Turkish air forces have been operating in the area where al-Adnani was believed to have been wounded, but it's unclear which military forces killed him.

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The U.S. — which had put a $5 million bounty on al-Adnani's head — had him under surveillance, the official said. The U.S. has been providing intelligence on targets to Turkey under a program called "Nomad Shadow," the official said.

Defense Dept. officials on Tuesday evening said they are still gathering information on the outcome of an assault on al-Adnani's position.

"Today coalition forces conducted a precision strike near Al Bab, Syria, targeting Abu Muhammad Al-Adnani, one of [ISIS]'s most senior leaders. We are still assessing the results of the strike, but al-Adnani's removal from the battlefield would mark another significant blow to [ISIS]," said Pentagon spokesman Peter Cook in a statement.

"The U.S. military will continue to prioritize and relentlessly target [ISIS] leaders and external plotters in order to defend our homeland, our allies and our partners, while we continue to gather momentum in destroying [ISIS]'s parent tumor in Iraq and Syria and combat its metastases around the world," he added.

Al-Adnani is best known for issuing an edict for lone wolves to kill Westerners in September 2014.

"If you can kill a disbelieving American or European — especially the spiteful and filthy French — or an Australian, or a Canadian, or any other disbeliever from the disbelievers waging war, including the citizens of the countries that entered into a coalition against the Islamic State, then rely upon Allah, and kill him in any manner or way however it may be," he said.