Mokhtar Belmokhtar was charged with leading a deadly gas-plant attack in Algeria

U.S. aircraft carried out a strike in Libya targeting a senior al Qaeda commander who masterminded terrorist attacks across the Sahara, the Pentagon said on Sunday.

While the Pentagon said it was still assessing results of the strike against Mokhtar Belmokhtar, the internationally recognized Libyan government said the militant had been killed.

Mr. Belmokhtar served as a senior commander in al Qaeda’s Saharan branch, before founding his own al Qaeda cell called the “Signed-in-Blood Battalion,” according to the U.S. National Counterterrorism Center.

In 2013, he led an attack on a gas facility in Algeria, resulting in a siege and the death of 38 hostages, including three U.S. citizens, the U.S. government said.

Trained in militant camps in Afghanistan, Mr. Belmokhtar in 2007 had been sentenced to death in absentia by an Algerian court.

“Belmokhtar has a long history of leading terrorist activities,” the Pentagon said, adding that he maintained a “personal allegiance to al Qaeda.”

Since the U.S.-backed ouster of Libyan strongman Moammar Gadhafi in 2011, that country has descended into a civil war marked by the rise of Islamist militant groups. It was also one of Mr. Belmokhtar’s favorite places to buy weapons, he has said.

The Libyan government said Mr. Belmokhtar was killed on Saturday night, the Associated Press reported. The Pentagon said it was assessing the outcome of the strike and would provide additional information “as and when appropriate.”

Mr. Belmokhtar has turned the region’s chaos to his advantage. Born in a mountainside town under dictatorial rule in Algeria, he left at the age of 19 for Afghanistan, where he claimed to have trained under Osama bin Laden and fought Soviet soldiers. But many U.S. officials and terrorist analysts said he arrived there too late to see combat and was too young to earn bin Laden’s tutelage.

He came back to Algeria missing an eye and with a long scar across his face, injuries he said came from battle shrapnel.

In the 1990s, as Algeria tipped into a civil war between Islamists and the state, Mr. Belmokhtar preached a more focused kind of terrorism. His allies were beheading children, gunning down entire villages, including Muslim towns, and committing widespread rape—religiously justified reprisals that foreshadowed acts by Boko Haram in Nigeria beginning a decade later.

Mr. Belmokhtar broke away, and helped form a group he said would spare reasonably observant Muslims.

But his Salafist Group for Preaching and Combat failed to gain traction and was reduced to a few dozen, hunger-stricken jihadists, forced into the barren south of Algeria. Defeated, he and his group snuck across the border, arriving in Mali, a West African democracy, around 2003, residents there said.

There, Mr. Belmokhtar turned to kidnapping, driving across the Sahara, snatching Europeans from hotels, and burying weapons, drums of gasoline, even trucks in the sand, his former hostages have said.

By 2006, their success in kidnapping and ransoming Europeans had caught bin Laden’s attention and al Qaeda began to accept the Mali-based group as an al Qaeda affiliate, named al Qaeda in the Islamic Maghreb, or AQIM. In letters to his other al Qaeda franchises, bin Laden described AQIM as his most profitable unit and encouraged more branches to take hostages—a tactic adopted by the al Qaeda militants in Iraq that eventually became Islamic State.

The U.S. and France took notice of him, too. The Central Intelligence Agency said it had been tracking him since the 1990s. Internet circles began to call him MBM, his initials. In Mali, locals called him Laaouar, or “the one-eyed.” AQIM called him Abu Khaled. The group called his followers the Masked Brigade.

Repeatedly, West African and European news outlets reported him dead. But in 2011, as Libya was celebrating Gadhafi’s removal—and beginning to slide into chaos—Mr. Belmokhtar told an al Qaeda-friendly press outlet that he was there, purchasing weapons from the ousted dictator’s looted arsenal.

In early 2012, he helped lead an army of Islamists that conquered northern Mali—including the storied caravan city of Timbuktu.

In January 2013, Mr. Belmokhtar’s group stormed the gas plant, took hundreds of hostages and killed dozens of them. “Now, you’re going to see what you’ve unleashed,” he said in a video claiming responsibility.

Write to Philip Shishkin at philip.shishkin@wsj.com and Drew Hinshaw at drew.hinshaw@wsj.com