America made clear last week that it would attack Taliban forces in their Swat valley stronghold unless the Pakistan government stopped the militants' advance towards Islamabad. London Times report quoted a senior Pakistani official saying the Obama administration intervened after Taliban forces expanded from Swat into the adjacent district of Buner, 60 miles from the capital. The Pakistani Taliban's inroads raised international concern, particularly in Washington, where officials feared that the nuclear-armed country, which is pivotal to the U.S. war against the Taliban in Afghanistan and against Al Qaeda, was rapidly succumbing to Islamist extremists. "The implicit threat - if you don't do it, we may have to - was always there," said the Pakistani official. He said that under American pressure, Pakistan's Inter-Services Intelligence agency told the Taliban to withdraw from Buner on Friday. However, reports Saturday indicated that the Taliban withdrawal was less than total. As a result, hundreds of thousands of people in the district were still at the mercy of armed militants and their restrictive interpretation of Islamic law. American military and intelligence forces already run limited ground and air operations on Pakistani soil along the border with Afghanistan. But an covert military operation such as that threatened in Swat, away from the border, would mark a major escalation. The official said last week's outspoken remarks by Hillary Clinton, the U.S. secretary of state, were "calculated to ramp up the pressure on Pakistan" to take action. Clinton warned that the terrorists' advance had created a "mortal threat" to world security. She was one of several American political and military leaders to use unusually strong language about Pakistan's failure to curb the Taliban. Admiral Mike Mullen, chairman of the joint chiefs of staff, who visited Pakistan, said he was "extremely concerned" about the developments and that the situation was "definitely worse" than two weeks ago. General David Petraeus, of US Central Command, which oversees Afghanistan - to which America is about to commit 17,000 more troops - said Al Qaeda and Taliban extremists in Pakistan posed an "ever more serious threat to Pakistan's very existence."