A top commander for the Islamic State was killed by U.S. commandos in a raid this week, the Pentagon said Friday, as the U.S.-led coalition ratcheted up pressure on the terrorist group in an effort to blunt its ability to strike targets abroad.

Defense Secretary Ashton B. Carter said the commander, known as Haji Imam, was one of several terrorist leaders killed during the same week in which the jihadists unleashed a lethal attack on the Belgian capital.

“We are systematically eliminating ISIL’s cabinet,” Carter said at a press briefing, using an acronym for the Islamic State. Despite a wave of terror inflicted on European cities by the group’s operatives, Carter asserted that the military campaign is having a punishing effect on the Islamic State in its Iraqi and Syrian strongholds.

“We’re certainly gathering momentum,” Carter said. “And we’re seeing that momentum is having an effect.”

The death of Imam is the second major blow to the Islamic State’s leadership ranks in a month, coming less than two weeks after Pentagon officials confirmed the killing of Abu Omar al-Shishani, the group’s top military commander.

Carter said the Islamic State leaders recently killed by the U.S. military had links to “external affairs” that extended the group’s reach beyond the “caliphate” it has tried to establish, although he was reluctant to connect the leaders to the recent terrorist attacks in Brussels and Paris.

Terrorism experts have long regarded Imam, an Iraqi native, as a kind of elder statesman for the terrorist group because of his long association with the Islamic State and its predecessor organizations. The Mosul native, reportedly about 57 years old, was one of the few surviving links to the era when al-Qaeda in Iraq, under the leadership of Abu Musab al-Zarqawi, soared to prominence at the forefront of the anti-U.S. insurgency in Iraq.

A U.S. official, who insisted on anonymity in describing military operations, described Imam as a “longtime member, someone who forms connective tissue to the al-Qaeda in Iraq days,” when the organization was led by Zarqawi, a Jordanian terrorist known for his brutal tactics. Zarqawi was killed in a U.S. airstrike in 2006.

U.S. officials stopped short of describing Imam as No. 2 in the organization, as he has been described by some analysts, saying that he held an array of important positions but remained largely behind the scenes. He did not have the high public profile of Islamic State leader Abu Bakr al-Baghdadi or Abu Mohammad al-Adnani, who functions as the group’s spokesman.

According to Hisham al-Hashimi, an Iraqi expert on Islamic State in Baghdad who advises the Iraqi government, Imam was a powerful and influential leader with a considerable following of his own in and around his home province of Nineveh, which includes Mosul.

He was a physics teacher from the Nineveh province town of Tal Afar, who became Baghdadi’s deputy in Iraq and Syria after a U.S. airstrike killed Abu Muslim al-Turkmani, a fellow Turkman also from Tal Afar, last year.

In the 1980s, he acquired Salafist ideology and was arrested by Saddam Hussein’s security apparatus on several occasions. In 1998, he traveled to Afghanistan and met Osama bin Laden.

After the U.S. invasion of Iraq, Imam returned to Tal Afar and founded a group called Saraya al-Jihad to fight the Americans. In 2004, he pledged allegiance to Zarqawi, a Jordanian terrorist known for his brutal tactics, and in 2008, he was named head of the Shura council under Abu Omar al-Baghdadi.

“He was known for his utmost loyalty to bin Laden” said Hashimi. In 2010 bin Laden appointed him to succeed Abu Omar al-Baghdadi. But the message regarding the appointment came two weeks after pledges of allegiance had been made to Abu Bakr al-Baghdadi, and it was too late.

There was a good chance he would have succeeded Baghdadi if Baghdadi were killed because of his history and his longstanding credentials as a Salafist jihadi who had been to Afghanistan and met bin Laden. “He is one of the old leaders in Al-Qaeda and he had a bigger past than Baghdadi,” Hashimi said.

However, although he had recently been boasting that he was a descendant of the Quraish tribe to which the prophet Muhammad belonged, his Turkman origins evidently meant he was not and may have precluded him from winning a position that was historically held by an Arab.

Terrorism experts have described him as charismatic and a gifted speaker as well as a skillful manager who played a critical role as a link between Baghdadi and the Islamic State’s regional “emirs” in Iraq and Syria. His death, coming on the heels of the killing of Shishani and other senior terrorist leaders, could undercut the terrorist group’s ability to coordinate its military operations, analysts say.

“The death of Haji Imam will surely cause problems for ISIS,” said Steve Stalinksy, executive director of the Middle East Media Research Institute.

There was no immediate reaction from the Islamic State’s official media, although jihadist social media sites noted the report of his death with skepticism. Iraqi officials had inaccurately reported that Haji Imam was killed last April during a U.S. airstrike in western Iraq.

His death could add further momentum to a military campaign that, by all accounts, is steadily weakening the Islamic State as a military force in Iraq and Syria. The terrorist group suffered new setbacks this week as Syrian government troops entered the outskirts of the historic town of Palmyra after a weeks-old offensive aided by Russian airstrikes. Separately, U.S. airstrikes helped Iraqi forces overrun a string of Islamic State villages in northern Iraq that had been threatening a U.S. base nearby.

Squeezed on multiple sides at once, Islamic State fighters have lost about 40 percent of the territory they once held, and they have not embarked on a successful offensive in nearly nine months. U.S. military officials say the group’s leaders are being killed at a rate of one every three days.

Some analysts say that the Islamic State’s attacks in France and Brussels are in part a reaction to its losses in its home territory.

Despite his prominence in the organization, relatively little is known about Imam’s background. His real name is Abd al-Rahman Mustafa al-Qaduli, according to the Treasury Department, which includes him on its list of Specially Designated Global Terrorists. But he has used a number of aliases over the years.

The Pentagon has targeted the leaders of groups such as al-Qaeda and the Islamic State, although senior leaders of these terrorist groups are often quickly replaced. “Striking leadership is necessary but far from sufficient,” Carter said.

Greg Miller and Greg Jaffe contributed from Washington. Sly reported from Beirut.

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