ISLAMABAD - Pakistan sent helicopter gunships and troops to attack Taliban militants Sunday in a district covered by a peace deal after strong U.S. pressure on the nuclear-armed country to confront insurgents advancing in its northwest.

At least 31 people were killed in the offensive, which sent some residents of Lower Dir district fleeing carrying small children and few belongings.

The operation appeared to endanger a peace pact struck with Taliban militants in neighbouring Swat Valley, although a top official insisted the deal was "intact." Another official demanded the insurgents disarm, but a Taliban spokesman said the militants would not give up their weapons.

The Lower Dir offensive also came ahead of Pakistani President Asif Ali Zardari's scheduled meeting with President Barack Obama in early May, but Pakistani officials denied outside pressure influenced the move.

Television footage from the district showed at least two helicopter gunships heading toward the mountains. Troops guarded a road blocked with paramilitary trucks, while some families sat nearby. Another family headed away in a vehicle packed with luggage.

The operation killed at least 30 militants, including a commander, plus one paramilitary soldier, according to an army statement and Interior Ministry chief Rehman Malik. The statement said the operation was launched at the request of the provincial government and local residents, but did not detail its scope or expected duration.

The government agreed to impose Islamic law in Swat and surrounding districts that make up Malakand Division if the Taliban there would end their violent campaign in the one-time tourist haven. Critics labelled the deal a "surrender" to the militants and warned Swat could turn into a haven for allies of al-Qaida.

In recent days, Taliban forces from Swat began entering Buner, a neighbouring district which lies just 100 kilometres from the Pakistani capital, Islamabad. Officials said most of the insurgents pulled out of Buner on Friday amid reports of possible military action, and threats that the government would scrap the deal.

Losing either Lower or Upper Dir to militants would be a blow not only for Pakistan but also for the U.S. because a part of the region borders Afghanistan, where the U.S. is sending thousands more troops to shore up the faltering war effort against a resurgent Taliban.