Updated

Pakistan launched a ground offensive against militant strongholds near the Afghan border on Monday after evacuating nearly half a million people from the region, the army said, in the most significant escalation of a two-week long operation to root out insurgents trying to overthrow the government.

The ground offensive is the second phase of a long-awaited operation against militants in the North Waziristan tribal area, a lawless, mountainous stretch of land in northwest Pakistan. The military announced the operation on June 15 but has mostly limited its tactics to airstrikes while giving time to hundreds of thousands of people to pack up their belongings and leave to safer areas.

The U.S. has long pushed for such an operation to go after militants that use the area as a safe haven from which to attack targets in both Pakistan and Afghanistan.

But for years Pakistan has said its forces were too strung out battling militants in other areas of the northwest to go into North Waziristan. The military is also believed to have been reluctant to launch the operation without political support from the civilian government. Until recently Prime Minister Nawaz Sharif had been pushing for negotiations over military force as a way to end the years of bloodshed caused by the militants.

The army began a house-to-house search in Miran Shah, the main town in North Waziristan, the army statement said. It said nearly 15 militants were killed in the initial ground advance. The town is also the headquarters for a number of different militant groups such as the Pakistani Taliban. Al-Qaida and the Afghan Taliban also have a presence in North Waziristan.

The operation began days after militants attacked the main airport in the southern port city of Karachi, killing 26 people. The 10 attackers also died in the roughly five-hour siege that shocked Pakistanis by showing how vulnerable the country's institutions have become.

The siege of the country's busiest airport became a turning point in the government's willingness to negotiate with the militants. A week after the attack, the military announced its troops were starting the North Waziristan operation.

Pakistani forces killed 376 militants during the first 15 days of the offensive, the statement said, adding that 17 soldiers also died. North Waziristan has always been a challenging area for journalists to access but the operation has made it even more difficult to independently verify reports of casualties.

The military said infantry and commandos are leading the ground advance. Three soldiers were wounded in an exchange of fire, the statement said.

Mansur Mahsud, from the FATA Research Centre which researches the tribal areas in northwestern Pakistan, said they had been receiving reports that many militants had left for neighboring Afghanistan or into the more remote mountainous areas in the northwest after the airstrikes. But he said a ground offensive was still necessary to clear the area.

In the past, critics have accused Pakistan of playing a double game, supporting or tolerating some militants that it sees as useful in maintaining influence in neighboring Afghanistan, and going after other militants that attack the Pakistani state. The military has said that this operation will go after everyone equally, but many question how aggressive they will be.

The operation could take three to four months, and it isn't likely to end militancy across the country immediately, said Mahsud. Militant groups still have a presence in places such as Karachi or Punjab province or other parts of the northwest.

But over time, Mahsud said it will significantly weaken the militants by denying them a place to headquarter their organizations and to train new recruits.

"It cannot end militancy 100 percent in Pakistan but it can have a significant effect," he said. "Once this area is cleared the militants are forced to shift to Afghanistan or the mountains."

About 468,000 people have poured out of North Waziristan, flooding the nearby Pakistani areas of Bannu and Dera Ismail Khan in anticipation of the ground offensive. An additional 95,000 went to Afghanistan, the United Nations reported.

The Pakistani army has already conducted several military operations in the tribal badlands along the Afghan border, including 2009 offensives in the scenic Swat valley and in South Waziristan, the onetime headquarters of the Pakistani Taliban.

The Pakistani Taliban is a loose network of several local militant groups who want to overthrow the country's government in a bid to install their own harsh brand of Islamic law. In their decade-old deadly campaign of bombings, shootings and other attacks, they have killed thousands of Pakistanis.

The government of Prime Minister Nawaz Sharif ever since he took office last summer had been trying to negotiate a peace deal with the militants. The operation has effectively ended prospects of any such move in near future.