Taliban militants said Monday their peace deal with the Pakistani government was "worthless" after authorities sent helicopters and artillery against hide-outs of Islamist guerrillas seeking to extend their grip along the Afghan border.

A collapse of the pact would likely please Obama administration officials pressing Islamabad hard for more robust action against extremists threatening the stability of nuclear-armed Pakistan as well as neighboring Afghanistan.

In another sign of mounting Western concern, British Prime Minister Gordon Brown was due in Pakistan for talks on topics including cooperating against international terrorism, the British Embassy said.

Pressure on the creaking peace deal grew Sunday when authorities sent troops backed by artillery and helicopter gunships to attack Taliban militants in Lower Dir, part of the region covered by the pact.

Paramilitary troops killed 20 suspected militants Monday, and a total of 46 had died since the operation began, an army statement said. Troops were combing the Maidan area of the district, it said.

Some terrified residents have fled the area clutching no more than their children and a few belongings. At least one soldier was killed Sunday.

A spokesman for the Taliban in their Swat Valley stronghold denounced the operation as a violation of the pact and said their fighters were on alert and waiting to see if a hard-line cleric who mediated the deal pronounced it dead.

"The agreements with the Pakistan government are worthless because Pakistani rulers are acting to please Americans," Muslim Khan, spokesman for Taliban militants in the Swat Valley, told The Associated Press.

A spokesman for Sufi Muhammad said the cleric was trapped in his home in the same area of Lower Dir attacked by troops Friday and that his supporters have been unable to contact him.

"We will not hold any talks until the operation ends," spokesman Izzat Khan said.

The government agreed in February 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.

In recent days, Taliban forces from Swat began entering Buner, a neighboring district just 60 miles (100 kilometers) from the Pakistani capital, stirring international alarm.

American officials have described the pact as a capitulation and urged Pakistani leaders to switch their security focus from traditional foe India to violent extremists inside their borders.