A top ISIS leader and spokesman thought to have a heavy hand in planning and ordering the group's international terror operations died in Syria, according to Amaq, ISIS' news agency.

"Shaykh Abu Muhammad al-Adnani, the spokesman of the Islamic State, was martyred while surveying the operations to repel the military campaigns in Aleppo," Amaq wrote, citing a military source, in a Tuesday news release sent to Telegram channels monitored by ISIS followers.

ISIS also declared revenge for the killing, saying that a generation raised in ISIS-held territory will take action, and that it is a generation where "the blood of the sheikhs will only make it more firm on the path of jihad and determination to take revenge and assault."

Adnani's death could not be immediately confirmed by U.S. officials.

“With regards to the reports about the death of ISIL leader Al-Adnani, I can confirm on background that earlier this morning (EDT) -- PM same day in Syria -- coalition forces conducted an airstrike in al Bab, Syria, targeting an ISIL senior leader," a senior defense official told Fox News. "We are still assessing the results of the operation at this time.”

The 39-year-old Adnani, born in Binnish in northern Syria, is described as having hand-selected the gruesome execution videos ISIS routinely puts out, according to The New York Times.

He was the subject of a $5 million State Department reward. In December, a senior intelligence source told NBC that Adnani was the member of ISIS -- not the group's leader Abu Bakr al-Baghdadi -- that the U.S. wanted dead the most.

"He is at the top of the list," the official said.

As the leader of the group's propaganda arm, Adnani was also the most vocal ISIS leader in calling for so-called "lone wolf" attacks against Western targets.

In a September 2014 speech Adnani encouraged Islamists to kill Europeans by any means necessary.

"Smash his head with a rock, or slaughter him with a knife, or run him over with your car," Adnani said.

In his most recent speech, in May, Adnani said attacks against civilians in Western countries were "dearer and better" for ISIS than success on the battlefield, according to SITE Intelligence Group, which monitors and analyzes the threat from jihadists.

Top ISIS fighters recruited into a secret elite fighting unit met directly with Adnani -- while blindfolded, so few knew what he looked like, The Times reported.

Fox News' Lucas Tomlinson contributed to this report