PESHAWAR, Pakistan — Not so long ago, Baitullah Mehsud was an obscure tribal sub-chieftain, little known outside his ancestral district set amid the forbidding, snow-shrouded mountains and valleys of South Waziristan.

Now, in a matter of months, he has emerged as the most notorious insurgent commander in Pakistan, blamed by authorities not only for masterminding Benazir Bhutto’s assassination but for waging a virtual one-man jihad against the government of President Musharraf.

Mr. Mehsud, who late last year became the leader of Pakistan’s Taliban movement, is accused of sending dozens of suicide bombers into Pakistani cities over the past year. He is also said to have unleashed a guerrilla campaign that has rattled Pakistan’s powerful military and brought pitched battles to the doorstep of Peshawar, capital of the volatile North West Frontier province and the gateway to Afghanistan and the tribal belt.

Some observers regard Mr. Mehsud as the most potent threat to emerge in years from the tribal milieu, a leader who has shown himself capable of unifying an array of disparate homegrown groups, even while exchanging crucial logistical aid, know-how and resources with Al Qaeda. If his coalition holds firm, these observers say, he could be in a position to threaten not only Mr. Musharraf but the Pakistani state. But the ascension of Mr. Mehsud also has prompted debate among analysts and military officials as to his true stature within the larger Al Qaeda-driven insurgency based in the tribal areas — a network whose strength and reach has been the subject of sharp new warnings this month by senior American intelligence officials.

Some observers see Mr. Mehsud as more of a figurehead, a handy scapegoat for a plethora of hostile acts unlikely to have been engineered by the same person. There are persistent suggestions, they say, that Mr. Mehsud is being used by elements within the Pakistani security establishment to further their own goals; before her death, Ms. Bhutto herself described him as a “pawn.”

Despite his growing infamy, Mr. Mehsud has preserved an aura of secrecy, speaking rarely to outsiders and scrupulously avoiding being photographed. Even his age is a mystery; he is believed to be in his 40s. Those who have met him describe a surprisingly unprepossessing figure. He stands only about 5 feet tall and wears his hair long and shaggy. Uneducated beyond the madrassa level, he is troubled by health problems stemming from diabetes.

“He wasn’t the imposing tribal type I expected,” Iqbal Khattak, a Peshawar-based journalist who has met Mr. Mehsud several times, said. “But in spite of his appearance, you could see that he has an authority about him.”

Both Mr. Mehsud and Al Qaeda vowed to take revenge against Mr. Musharraf after government forces in July stormed a radical mosque in the capital, killing its chief cleric and dozens of his young disciples, many of whom were from South Waziristan.

Estimates of the size of Mr. Mehsud’s corps of fighters vary. His inner circle of several hundred armed followers is mainly drawn from his tribe, whose reputation as a feared fighting force dates to British colonial times.

In mid-December, however, Mr. Mehsud assumed leadership of an umbrella group known as the Tehrik-i-Taliban Pakistan, or the Taliban movement of Pakistan, encompassing at least half a dozen militant groups and upward of 20,000 fighters, officials believe. “He’s a very clever and committed fighter, a good tactician, with no qualms at all about extreme forms of violence” such as beheadings, Mehmood Shah, a retired brigadier general with experience in the tribal areas, said.

Concerns about the threat posed by both Al Qaeda and Mr. Mehsud’s new coalition have prompted some of the Bush administration’s most senior intelligence and military officials to travel to Pakistan to lobby for more aggressive American military action. “This is a threat to the identity and stability of the Pakistani state,” CIA Director Michael Hayden has said.