Updated

Pakistani jets pounded targets in the country's northwest on Monday as the military waged an offensive against militant safe havens that killed 37 fighters, while insurgents fought back with a roadside bomb that killed six soldiers, the first army casualties of the operation, the military said.

The airstrikes are part of a long-awaited operation against foreign and local militants in North Waziristan, a tribal region near the Afghan border that is used by insurgents to attack the rest of Pakistan. The operation was first announced on Sunday.

The essentially lawless region serves as a training base for militants and a staging point for insurgents who attack NATO and Afghan troops across the border.

The United States has for years complained to Pakistan about its failure to impose order in a region harboring what Washington considers the country's most dangerous militants, and has carried out hundreds of drone strikes in the area.

The airstrikes early Monday targeted six hideouts in the Shawal area near the border with South Waziristan, a neighboring tribal region, killing 27 militants, the military said.

Separately, the military said seven militants were killed trying to escape Mir Ali, one of the two main towns in North Waziristan, and another three were killed by snipers while trying to lay roadside bombs near Miran Shah, the region's other main town. Three Pakistani troops were wounded in an exchange of fire with militants.

The military said in a statement that six troops died and three were wounded by the roadside bomb as the Pakistani Taliban warned that more violence could be in the offing.

A spokesman for the militants warned in a statement sent to reporters that international investors, foreign airlines and multinational organizations should leave Pakistan or they'd be considered supporters of the government and fair game.

Shahidullah Shahid also vowed to retaliate in the cities of Islamabad and Lahore, saying they would burn the mansions of the country's rulers. Those cities have largely been spared the bombings and shootings that plague cities like Karachi and Peshawar.

Few details have been released about the size of the operation or the military assets involved.

The military said troops were sealing off the North Waziristan borders and the main cities, ensuring the evacuation of civilians in addition to establishing areas where militants can surrender their weapons. The military said the campaign was progressing as planned and that no operations in civilian areas had been started so far.

Residents reached by telephone reported hearing loud explosions overnight but said they cannot go outside because there is a curfew in place.

"We have been just hearing big bangs and explosions. We can't go out to see what's happening out there," Sajid Dawar, a resident of Miran Shah, said by telephone. He urged authorities to help people leave as quickly as possible.

Another resident, Ziaullah Khan from Mir Ali, said people were starting to run out of food because the markets had been closed for a few days.

The North Waziristan tribal area is one of the last parts of the tribal regions where the military has not launched a large operation. Militant groups including the Pakistani Taliban, al-Qaida and the Haqqani network have long used the region as a base.

The Pakistani military said it has asked the Afghan government for help reinforcing its side of the border, to prevent militants from fleeing.

Afghanistan and Pakistan have argued repeatedly over violence along the porous border, accusing each other of failing to act against insurgents.

Kabul accuses Pakistan of failing to take action against militants such as the Haqqani network, who are believed to be responsible for some high-profile attacks in Afghanistan.

Critics also say Pakistan maintains a so-called "good Taliban, bad Taliban" policy in which it combats the Pakistani Taliban, who attack the state, but tolerates or supports the Afghan Taliban in order to maintain influence in the neighboring country.

The offensive marks the end of the government's policy of trying to negotiate with Pakistani Taliban militants instead of using force to end years of fighting which has killed tens of thousands of civilians and security forces.

It comes a week after the militants laid siege to Pakistan's largest airport in an attack that shocked the country and appeared to mark a turning point in the government's thinking about the offensive.

But the large military operation could spark a wave of bloody reprisal attacks, especially in places such as Karachi or Peshawar, where militants already have a sizeable presence.

Even before the airstrikes, residents in North Waziristan were leaving the area due to previous airstrikes and out of fear of a larger operation.

Nawal Khan Dawar from Mir Ali said he arrived in Bannu six days ago with his family after an air strike nearby destroyed several houses. He said he rented a house for his family but, without any work, he doesn't know how long they'll be able to stay. He said he's worried about other family members still in North Waziristan.

"We don't know what happening with them. We have not heard from them," he said.

Some of the displaced were even going to Afghanistan. Mubariz Mohammad Zadran, a spokesman for the provincial governor of Afghanistan's Khost province, said over the last two weeks at least 404 families have entered from North Waziristan.