WASHINGTON (AP) — The civilian official overseeing the Pentagon's campaign to defeat the Islamic State group in the Middle East was forced to resign in the latest jolt to Pentagon leadership in the waning weeks of the Trump administration.

The Pentagon said in a written statement that the acting defense secretary, Christopher Miller, on Monday accepted the resignation of Christopher Maier, who had provided policy oversight of the military's counter-IS effort since March 2017.

A defense official familiar with the matter said Maier was told Monday that since President Donald Trump had long ago declared the IS militant group defeated, his office was being disbanded and he was abruptly “terminated.” The official spoke on condition of anonymity to discuss an internal personnel matter.

Maier, a career counterterrorism official, was director of the Defeat-ISIS Task Force, whose responsibilities are to be absorbed by counterterrorism staffs headed by appointees who President Donald Trump placed in senior Pentagon positions in a shakeup that included his firing of Defense Secretary Mark Esper on Nov. 9.

Maier's departure was first reported by CNN. The New York Times was first to report that Maier had been forced out.

In its statement, the Pentagon gave no reason for Maier's departure but said the decision to disband the task force he led was a recognition of the “success of the military fight to destroy” the Islamic State's grip on territory in Iraq and Syria. Critics say that while the militant group has lost its physical empire, it remains a threat and has been biding its time in search of ways to regroup and re-emerge.

“The Department of Defense will continue to engage with our partners and allies to ensure the lasting defeat of ISIS and encourage the repatriation of foreign terrorist fighters for prosecution,” the Pentagon said.

Nearly 900 U.S. troops are still in Syria to work with local groups aiming to prevent an IS resurgence. The U.S. also has about 3,000 troops in neighboring Iraq working with local security forces toward the same goal.

The counter-IS campaign began during the Obama administration and in some respects was accelerated by Trump.