MINGORA, Pakistan —Residents flooded out of the Swat valley by the thousands Tuesday as the government prepared to mount a new military campaign against Taliban militants, and a much-criticized peace accord with the insurgents unraveled.

Black-turbaned militants seized government buildings, laid mines and fought security forces Tuesday in the Swat valley, as fear of a major operation led thousands to pack their belongings on their heads and backs, cram aboard buses and flee the northwestern region.

The collapse of a three-month-old truce with the Taliban means Pakistan will now have to fight to regain control of the Swat valley, testing the ability of its stretched military and the resolve of civilian leaders who until recently were insisting the insurgents could be partners in peace. The government feared the refugee exodus could reach 500,000.

The developments brought Islamabad’s faltering campaign against militancy into sharp focus as President Asif Ali Zardari was preparing for talks today in Washington with President Barack Obama and Afghan President Hamid Karzai on how best to counter an increasingly overlapping spectrum of extremist groups behind surging violence in the neighboring countries.

The Obama administration hopes to build a strong and lasting regional alliance, linking success in Afghanistan with security in Pakistan. Toward that end, the administration is encouraging Pakistan to confront — not make peace with — the Taliban and other militants.

“We need to put the most heavy possible pressure on our friends in Pakistan to join us in the fight against the Taliban and its allies,” Richard Holbrooke, the U.S. special representative for Afghanistan and Pakistan, told a congressional committee Tuesday. “We cannot succeed in Afghanistan without Pakistan’s support and involvement.”

It is far from certain that the Pakistani public has the stomach for a long battle in Swat. Given that the militants have had time to rest and reinforce their positions in the three months since the truce took effect, any operation would involve fierce fighting in an urban setting and almost certainly cause significant civilian casualties and damage to property.