But the Taliban quickly dismissed any talk of their leader’s being dead. And other Afghan officials, along with some European and American counterparts, said the suggestions that Mullah Omar had died were a propaganda ploy intended to weaken Taliban morale, not a reflection of the true thinking within the Afghan government.

“There’s a consensus among all three branches of the Afghan security forces that Mullah Omar is alive,” said one European official, who spoke on the condition of anonymity to discuss private intelligence briefings. “Not only do they think he’s alive, they say they have a good understanding of where exactly he is in Karachi,” the Pakistani metropolis where some say Mullah Omar is hiding.

Mullah Omar has always functioned more as the spiritual and ideological leader of the movement than as an operational commander. His inner circle, made up of village mullahs who have known one another for decades, has provided the active leadership of the Taliban’s many local factions.

But now one man, Mullah Akhtar Muhammad Mansour, has risen to the No. 2 role and become the main link to Mullah Omar, allowing him to place his loyalists up and down the ranks, Mr. Nabil said.

Mr. Nabil said that, in one example, a prominent commander operating south of Kabul was killed by fellow Taliban fighters in May or June because he was close to a rival of Mullah Mansour.

Mr. Nabil, in his assessment, saw Pakistan’s security establishment driving the changes, an appraisal shared by some Western officials. Some said it was a bid for greater control over the insurgency; others saw it as the evolution of a long-running Pakistani effort to avoid the embarrassment of having Mullah Omar discovered being sheltered in their country.

“There is a lot of doubt whether he is alive or not,” Mr. Nabil said. But “we are more confident that he is in Karachi.”