(Updates toll, adds police comment)

By Zeeshan Haider

ISLAMABAD, July 15 (Reuters) - Forty people were killed in northwest Pakistan on Sunday in a surge of militant violence which officials said could be aimed at avenging the commando assault on a radical mosque in the capital last week.

Also on Sunday, pro-Taliban militants in the North Waziristan region on the Afghan border called off a 10-month peace deal with the government after accusing authorities of violating the pact.

About 90 people, most of them paramilitary soldiers and police, have been killed in attacks in the northwest since July 3, when security forces in Islamabad surrounded the Lal Masjid, or Red Mosque, complex following clashes with gunmen.

Commandos stormed the fortified mosque-school compound a week later killing 75 supporters of hardline clerics, most of them militant gunmen.

Early on Sunday, 14 people, 11 of them paramilitary soldiers, were killed in a suicide-bomb ambush on a patrol in the scenic Swat valley in North West Frontier Province (NWFP).

Hours later, a suicide bomber targeted a police recruiting centre in the city of Dera Ismail Khan, in the same province, killing 26, many of them young men taking a police entrance exam, police said. Dozens were wounded.

Interior Minister Aftab Ahmed Sherpao said the two attacks could be a militant response to the Lal Masjid assault.

"It's very difficult to stop suicide attacks," Sherpao told Geo TV.

Security analysts had expressed fears of a militant backlash over the Lal Masjid assault.

Many of the militants at the mosque and many of the religious students who studied at the complex, were believed to have been from the NWFP.

Pakistan's rugged northwest is a hotbed of al Qaeda and Taliban support, U.S. military officers in Afghanistan say.

PEACE DEAL ABANDONED

In the Dera Ismail Khan attack young men were waiting to have their documents checked before the police entrance exam when the bomber struck, witnesses said.

"He was standing in a line with the recruits and all of a sudden blew himself up," said police official Inayatullah Amjad.

In the Swat valley attack, two suicide bombers rammed cars into a security force convoy as a roadside bomb went off. Three civilians were also killed, said military spokesman Major-General Waheed Arshad. About 30 soldiers were wounded.

Twenty-four paramilitary soldiers were killed in a suicide car bomb attack in North Waziristan on Saturday, in the most serious single attack on security forces since November.

The collapse of the North Waziristan peace deal did not appear to be linked to the Lal Masjid assault but is likely to add to the problems security forces are facing.

Under the pact, authorities agreed to stop operations against militants in return for their pledge to not send fighters into Afghanistan or launch attacks on security forces.

While U.S. military officials in Afghanistan said the pact had not stopped insurgent raids into Afghanistan, it did lead to a sharp fall in attacks on Pakistani forces in North Waziristan.

A militant leadership council said it was abandoning the pact because security forces had launched several attacks on them and the government had deployed more troops in the region.

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