WASHINGTON, D.C. — The Islamic State (ISIS/ISIL) core in Iraq and Syria is losing territory, fighters, leaders, and financial resources, while its Syrian rival Jabhat al-Nusra flourishes, becoming the “largest” al-Qaeda affiliate “in history,” President Barack Obama’s special envoy for the U.S.-led coalition against ISIS told lawmakers.

Brett McGurk, the envoy, testified before the Senate Committee on Foreign Relations Tuesday.

The U.S.-led coalition against ISIS, which has been bombing Syria since 2014, has primarily targeted ISIS.

U.S. military officials have conceded that the air campaign against ISIS has helped Jabhat al-Nusra, also known as the al-Nusra Front, gain strength and become more potent.

In his written testimony, McGurk said:

In Syria, as ISIL is losing territory in the east, its terrorist rival – Jabhat al-Nusra – is gaining ground in the west, putting down roots in Idlib province along the Turkish border. Nusra is establishing schools and training camps, recruiting from abroad, launching major military operations, and enjoying a sophisticated online presence, all the while providing safe haven for some of al Qaida’s most experienced terrorists. With direct ties to Ayman al Zawahiri, Osama Bin Laden’s successor, Nusra is now al Qaida largest formal affiliate in history.

This is a serious concern, and where we see Nusra planning external attacks, we will not hesitate to act.

His testimony comes less than a month after the U.S. State Department reported that al-Qaeda in the Arabian Peninsula (AQAP) , the Yemen-based affiliate of the jihadist group, quadrupled its manpower from “approximately 1,000 members” in 2014 to “4,000” last year.

The State Department report noted that the strength of the Nusra Front in 2015 is unknown.

A Reuters investigation published in April found that AQAP, which has expanded its reach to unprecedented levels, has also “become stronger than at any time since it first emerged almost 20 years ago.”

Moreover, CNN quoted Afghan Defense Minister Masoom Stanikzai as saying in April that core al-Qaeda in Afghanistan is “very active” and is “preparing themselves for bigger attacks.”

The group has doubled “their resources and recruitment and other things,” also said the Afghan defense minister, adding that it poses a “big threat.”

Top U.S. military officials indicated that al-Qaeda had “renewed” its partnership with the Taliban, adding that the relationship between the two 9/11-linked groups has “grown stronger.”

“Now, al-Nusra Front and [ISIS] don’t get along… I guess you could say to the extent that we’re weakening [ISIS], maybe it benefits al-Nusra Front,” Lt. Gen. Sean MacFarland, the top U.S. commander in Iraq , conceded during a press conference in February.

McGurk revealed that while the al-Nusra Front is growing stronger, the number of ISIS fighters in Syria had dropped to its lowest levels since the group swept through the region in the summer of 2014.

“We currently estimate that ISIL fields 18-22,000 overall fighters in Iraq and Syria, the lowest assessed range since we began conducting rigorous reviews of its manpower,” he told lawmakers. “This is down from a high-end estimate of 33,000 ISIL fighters in 2014. We are also seeing significant reduction in the flow of foreign fighters entering Syria and Iraq each month.”

He also noted that ISIS is losing territory, reiterating that the group “has lost 47 percent of its territory in Iraq, and 20 percent in Syria.”

Furthermore, McGurk pointed out that the U.S.-led coalition has degraded ISIS’s sources of revenue, particularly its oil infrastructure and cash storage sites.

“We have killed more than 100 mid-to-senior level ISIL leaders in the past few months alone,” he also said, adding, that the coalition is “succeeding” in countering ISIS propaganda in cyberspace.

“Whereas ISIL two years ago had nearly free rein in cyberspace, today, there are reportedly six people opposing ISIL’s message online for each person supporting it,” declared McGurk.