Offering the most definitive public assessment by a US intelligence official, Hayden said Bhutto was killed by fighters allied with Mehsud, a tribal leader in northwestern Pakistan, with support from al-Qaeda’s terrorist network. That view mirrors the Pakistani government’s assertions.

The same alliance between local and international terrorists poses a grave risk to the government of President Pervez Musharraf, a close US ally in the fight against terrorism, Hayden said in an interview with The Washington Post.

“What you see is, I think, a change in the character of what’s going on there,” he said. “You’ve got this nexus now that probably was always there in latency but is now active: a nexus between al-Qaeda and various extremist and separatist groups.” Hayden added, “It is clear that their intention is to continue to try to do harm to the Pakistani state as it currently exists.”

Days after Bhutto’s assassination, Pakistani officials released intercepted communications between Mehsud and his supporters in which the tribal leader praised the killing and, according to the officials, appeared to take credit for it. Pakistani and US officials have declined to comment on the origin of that intercept, but the administration has until now been cautious about publicly embracing the Pakistani assessment.

Many Pakistanis have voiced suspicions that Musharraf’s government played a role in Bhutto’s assassination, and Bhutto’s family has alleged a wide conspiracy involving government officials. Hayden declined to discuss the intelligence behind the CIA’s assessment, which is at odds with that view and supports Musharraf’s assertions.

“This was done by that network around Baitullah Mehsud. We have no reason to question that,” Hayden said. He described the killing as “part of an organised campaign” that has included suicide bombings and other attacks on Pakistani leaders.

Some administration officials outside the agency who deal with Pakistani issues were less conclusive, with one calling the assertion “a very good assumption”. One of the officials said there was no “incontrovertible” evidence to prove or rebut the assessment.

Hayden made his statement shortly before a series of attacks occurred this week on Pakistani political figures and army units. Pakistani officials have blamed them on Mehsud’s forces and other militants.

For more than a year, US officials have been nervously watching as al-Qaeda rebuilt its infrastructure in the tribal regions along the border between Afghanistan and Pakistan, often with the help of local sympathisers.

In recent months, US intelligence officials have said, the relationship between al-Qaeda and local insurgents has been strengthened by a common antipathy toward the pro-Western Musharraf government. The groups now share resources and training facilities and sometimes even plan attacks together, they said.

“We’ve always viewed that to be an ultimate danger to the US,” Hayden said, “but now it appears that it is a serious base of danger to the current well-being of Pakistan.”

Hayden’s anxieties about Pakistan’s stability are echoed by other US officials who have visited Pakistan since Bhutto’s assassination. White House, intelligence and Defence Department officials have held a series of meetings to discuss US options in the event that the current crisis deepens, including the possibility of covert action involving Special Forces.