As the self-proclaimed Islamic State trumpets its global terrorist campaign, U.S. special operations forces have quietly killed more than three dozen key ISIS operatives blamed for plotting deadly attacks in Europe and beyond.

Defense officials tell The Daily Beast that U.S. special operators have killed 40 “external operations leaders, planners, and facilitators” blamed for instigating, plotting, or funding ISIS’s attacks from Brussels and Paris to Egypt and Africa.

That’s less than half the overall number of ISIS targets that special operators have taken off the battlefield, one official explained, including top leaders like purported ISIS second-in-command Haji Imam, killed in March.

The previously unpublished number provides a rare glimpse into the U.S. counterterrorist mission that is woven into overall coalition efforts to defeat ISIS, and which is credited with crippling ISIS efforts to recruit foreign fighters and carry out more plots like the deadly assault on Paris that killed 130 last fall.

As proof of the campaign’s overall success, Pentagon officials this week said the overall size of ISIS from a high estimate of 33,000 a year ago to between 19,000 to 25,000 fighters, and that the influx of foreign fighters into Iraq and Syria had dropped from up to 2,000 a month last year to just 200. Secretary of Defense Ash Carter was more cautious about that figure in testimony Thursday morning, saying it is “hard to be accurate” estimating foreign fighter flow, but that the numbers generally are falling. That’s set against the warning by Director of National Intelligence Jim Clapper this week that ISIS cells are likely already in place across Europe.

That’s set against the warning by Director of National Intelligence Jim Clapper this week that ISIS cells are likely already in place across Europe.

The U.S. strikes have picked up pace since Defense Secretary Carter announced the deployment of special operations forces to northern Iraq last December, under the unwieldy moniker of “Expeditionary Targeting Force,” the officials said. They spoke on condition of anonymity because they were not authorized to describe the special operations mission publicly.

The officials expect that tempo to rise as the newly expanded special operations advising team inside Syria also grows from 50 to up to 300, as President Obama announced in Germany on Monday.

Officials say the Syria-based U.S. special operators help stitch together the disparate members of the Syrian Defense Force and vet others who want to join the mission, while also gathering intelligence on the ground that leads to strikes.

The CIA, NSA, and other elements of the U.S. intelligence community are also driving the effort, finding and feeding the intelligence to the coalition strike force.

At the top of the special operations target list is the network of ISIS operatives blamed for “external operations”: 60 attacks in 21 countries that have killed 1,000 people since January 2015, the officials said. Most of the ISIS targets were killed in Syria, by special operations combat aircraft, but also by troops who attempted to capture a handful of high-value ISIS targets in raids. All of those targets resisted arrest and were killed, the officials said.

That grim tally includes the previously announced December killing of Syrian-based ISIS member Charaffe al-Mouadan, who officials have concluded had direct ties to Abdel Hamid Abaoud, the leader of the ISIS cell that attacked Paris last November. Mouadan was among an estimated 10 militants taken out in a spec-ops airstrike.

Another was Abdul Kader Hakim, killed in Mosul in December. The Pentagon called Hakim an “external operations facilitator” and a forgery specialist with links to the Paris attack network.

It’s not clear how many civilians may have been caught in the special operations-related strikes. The U.S. has admitted to accidentally killing 41 civilians in the 20 months since coalition strikes began.

Sometimes the kills or attempted captures are not announced, in order to see how ISIS responds, one of the senior officials explained. “What are they doing, what are they saying, who are they communicating to? How do they backfill the missing operator?” he said. Those reactions can reveal weakness the U.S. task force can exploit.

“The point of such operations is to keep ISIS guessing,” he said.

Defense officials acknowledge the downside of the secrecy of the operations is that humanitarian and human-rights organizations that try to serve as neutral arbiters in war zones don’t always know who to call when civilians report allegations of casualties or damage in the aftermath of a military strike—or when someone goes missing, possibly taken in a raid. Two senior defense officials said they were actively working to establish and maintain relationships with such agencies in areas where their troops operate, including sharing with the International Committee of the Red Cross details of any detainees taken within a short time of their capture, as per Pentagon policy on detainees.

“Defense regulations… stipulate that information concerning detainees in U.S. military custody should be provided to the ICRC normally within 14 days,” ICRC spokesman Anna Nelson said. “In practice, as soon as we are made aware of a new detainee in U.S. custody, we will get in contact with the U.S. authorities to organize a visit.”

The special operations counterterrorist mission is spearheaded by troops from the Joint Special Operations Command, the U.S. military’s premier counterterrorist unit.

But unlike previous conflicts, where JSOC raiders worked in secret, usually apart from other types of special operators, the Iraq and Syria teams blend specialists from multiple disciplines. “Door kickers” from units like the U.S. Army’s Delta Force and the Navy SEALs’ Naval Special Warfare Development Group who train for hostage rescue missions or kill-capture raids are paired with operators like Green Berets who specialize in learning foreign languages and cultures, and training local forces.

“The teams are integrated in just about everything we do,” one defense official said.

The mixing of troops may have something to with the background of those in charge of the ISIS fight. Current JSOC commander Lt. Gen. Austin S. Miller and his predecessor, Gen. Tony Thomas, both ran the overall special operations task force in Afghanistan, which blended the different skills of very different, sometimes competing spec-ops tribes.

Thomas now runs the U.S. Special Operations Command. Miller most recently commanded Fort Benning, Georgia, where he oversaw the U.S. Army Ranger School that produced the first successful women candidates ahead of the Pentagon’s decision to open all combat roles to women.

And Gen. Joseph Votel, who previously led both USSOCOM and JSOC, now runs the ISIS campaign as head of Central Command. While rooted in the counterterrorism realm earlier in his career, he has a broader perspective on what the different special operations tribes bring to the fight.

Updated 4/28/16 10:30 a.m. to add the director of National Intelligence’s warnings that ISIS cells are likely already deployed to Europe, coalition airstrike casualty figures, and comment from the ICRC.