WASHINGTON — The reported death of the Islamic State’s senior propagandist and strategist, Abu Muhammad al-Adnani, in an American drone strike in northern Syria on Tuesday casts in sharp relief the immediate challenge the terrorist group faces in replacing one of its pivotal founding members.

The attack, carried out by a military Reaper drone, also underscores the progress the military’s most elite Special Operations commandos and the Central Intelligence Agency have made in the conflict’s two years by using information from spies on the ground and sensors in the sky to target a growing number of Islamic State leaders.

The American-led coalition has killed about 120 important Islamic State officials and operators, including about a dozen of the group’s top leaders, according to the Pentagon.

Still, the Islamic State has proved to be remarkably resilient, American officials and counterterrorism specialists say, noting that the group, also known as ISIS or ISIL, has succession plans to replace even its top leader, Abu Bakr al-Baghdadi, should he be killed.