London, Apr 26 : The United States made it clear to Pakistan that it will attack Taliban militants in the Swat valley if the Pakistani Government failed to stop their advance towards Islamabad.

A senior Pakistani official said the Obama administration intervened after Taliban forces expanded from Swat into the adjacent district of Buner, 60 miles from the capital.

The Tehreek-e-Taliban Pakistan inroads in the Swat valley raised international concern, particularly in Washington, where officials feared that Islamabad was rapidly succumbing to Islamist extremists, The Times reported.

"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 (ISI) told the Taliban to withdraw from Buner on Friday.

However, reports on Saturday indicated that the Taliban did not fully withdraw from the area. As a result, hundreds of thousands of people in the district were still at the mercy of armed militants.

American military and intelligence forces already run limited ground and air operations on Pakistani soil along the border with Afghanistan. But an overt 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 US Secretary of State Hillary Clinton 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.

Chairman of the Joint Chiefs of Staff Admiral Mike Mullen, 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, said al-Qaeda and Taliban extremists in Pakistan posed an ever more serious threat to Pakistan's very existence. (ANI)