"This was not a DOD operation," a Pentagon official said Saturday. Spokespeople for the CIA and National Security Council did not immediately respond to requests for comment.

AQAP has not issued a statement of its own confirming Rimi's killing.

Some analysts had considered Rimi to be a possible successor to Ayman al-Zawahiri, the leader of al Qaeda's overall umbrella operation. In 2006, Rimi and several other AQAP members escaped from a prison in Yemen -- one of several failed attempts to take him off the battlefield.

"He was/is an important figure for sure, but it’s important not to overestimate the impact of taking out guys like him," said Thomas Joscleyn, a terrorism expert at the Foundation for Defense of Democracies, who noted that others were waiting to replace him.

"It’s necessary to degrade these groups but not sufficient to defeat them," he added, "and I don’t think anyone has a good solution for the latter."

"If it is true," said Yemen expert Gregory Johnsen, who noted that the U.S. had yet to confirm Rimi's death, "it means that the end of AQAP as an international terrorist threat is in sight. The group is in disarray and [Rimi] was the last leader with strong ties to the group's founding in 2009."

Rimi's death, if confirmed, would add to a string of recent successes for U.S. counterterrorism operations. In early January, the U.S. used a Reaper drone to wipe out Qassem Soleimani, an Iranian general deemed responsible for dozens of attacks on American troops in Iraq.

And last year, U.S. special forces troops killed Abu Bakr al-Baghdadi, the leader of the Islamic State terrorist group or ISIS, in a raid inside Syria.

But senior U.S. officials have warned that ISIS is making a comeback in Iraq and Syria, blaming last year's incursion by Turkey for breathing new life into a group that President Trump has claimed has been 100 percent eliminated.

"We are seeing ISIS come back as an insurgency, as a terrorist operation, with some 14- to 18,000 terrorists between Syria and Iraq," ISIS envoy James Jeffrey said in a State Department briefing on Friday.

And Yemen -- in the throes of a civil war that has drawn in Saudi Arabia, the United Arab Emirates and Iran -- remains so dangerous that the United States Embassy there has been closed for years.

Earlier this week, Secretary of State Mike Pompeo said the U.S. government was "alarmed" by a recent uptick in violence in Yemen, which he said "produces instability that terrorist groups and other malign actors can exploit for their own purposes."

In 2017, after a botched U.S. special forces raid against AQAP led to the first American combat death of Trump’s presidency, Rimi taunted the new U.S. leader in an 11-minute audio recording.

"The new fool of the White House received a painful slap across his face,” Rimi said at the time.