People based in North and South Waziristan along Pakistan´s border with Afghanistan told The Washington Times that the top Pakistani Taliban commander, Baitullah Mehsud, and two rival Taliban chiefs, Hafiz Gul Bahadur and Maulvi Nazeer, met at an undisclosed location recently and settled their differences to unite against U.S. and Pakistani government operations in the region.

U.S. counterterrorism officials confirmed the account. They, like the Pakistanis, requested anonymity because of the nature of their work.

The unification appears, in part, to reflect U.S. success in killing al Qaeda and other militant leaders in Pakistan with attacks by unmanned aircraft. But the consequence is that it will be easier for Taliban fighters in southern and eastern Afghanistan to cross into Pakistan and elude U.S.-led forces, said an Afghan official who also spoke on the condition of anonymity.

"This certainly will place a strain on NATO efforts and Afghanistan's ability to gain control of the border region," the Afghan official said. "This changes everything."

The new Taliban alliance announced allegiance to Afghan Taliban chieftain Mullah Mohammed Omar as their "supreme leader" in the fight against U.S.-led forces, the Afghan official said.

Afghan Taliban commander Sirajuddin Haqqani, whose organization has mounted numerous attacks on U.S. and allied security forces, apparently influenced the Pakistani Taliban to unite in response to stepped-up U.S. attacks on Taliban and al Qaeda militants in Pakistan.

The Pakistanis, who asked not to be named to protect themselves from Taliban reprisals, said the unified group is calling itself the Shura-e-Ittehadul Mujahedeen (Council of United Holy Warriors).

Mr. Haqqani has attempted for years to unify Pakistani Taliban organizations.

Earlier this month, Pakistan's civilian government agreed to a truce with a Taliban faction in the Swat Valley adjoining the tribal areas. Under the agreement, the Taliban can implement Islamic law in the area, once a resort destination for secular Pakistanis.

A U.S. counterterrorism official said Pakistani government "action on the ground to this point has not been enough" to quell the Taliban influence.

"It's hard to overstate the complexity of the challenge Pakistan faces from these groups," said the official, who asked not to be named because of the sensitive nature of his work.

Terrorism specialists in Peshawar said the Taliban factions are consolidating to deal with intensified and ongoing U.S. missile strikes that have killed numerous militants over the past seven months.

Gul Marjan Wazir, an analyst from the University of Peshawar, who is originally from the Wana area in South Waziristan, said the three commanders first considered uniting after the U.S. began attacks by drone aircraft in 2008. The strikes hit an extremist training facility controlled by the Shbikhel tribe, a sub-tribe of Mr. Mehsud's Tehrik-e-Taliban, which is estimated to have about 10,000 fighters.

Known as TTP, the organization formed in 2007 and is an umbrella for militants from various Pakistan tribal agencies and some parts of the North West Frontier Province.

Al Qaeda No. 2 Ayman al-Zawahri reportedly has been seen at Mesud training camps in South Waziristan.

Another militant commander, Maulvi Nazeer, was injured in a U.S. drone strike in October in Dhog village near Wana.

Ashraf Ali, a senior researcher and writer on the Taliban, also underlined the importance of the unification deal. He suggested that rogue elements of Pakistan's Inter-Services Intelligence agency may have aided Mr. Haqqani in bridging differences among the militants.

"There could be reasons to believe that some elements within the Pakistani intelligence networks might have secretly helped Taliban groups to remove their differences," Mr. Ali said. "By doing so, either Pakistan may want to pressurize the U.S. to stop drone attacks or to justify these strikes domestically and by telling the world that Taliban are mending fences and becoming a threat to Pakistani state."

A Pakistani official in Washington denied involvement by members of his country's intelligence service toward unifying the Taliban. He asked not to be named because of the sensitive nature of the issue.

Mr. Gul Bahadur, from North Waziristan, and Mr. Nazeer were once considered to be supporters of the Pakistani government, which gave them weapons to oppose Mr. Mehsud.

Numerous bloody clashes ensued, primarily over Mr. Mehsud's decision to harbor Uzbek militants.

• Sara A. Carter reported from Washington.