Pakistani security forces have entered the main town in a northwestern Taliban stronghold, engaging in fierce street battles Saturday as they tried to wrench the Swat Valley from militants, the army said.

Capturing Mingora town is critical to Pakistani efforts to regain the valley and prevent it from being a safe haven for insurgents who threaten the nuclear-armed Muslim nation's stability. It also could prove a major test for a military more geared toward conventional warfare on plains than bloody urban warfare.

The military operation has strong support from Washington, which wants Pakistan to root out insurgents who are using hide-outs in the northwest to plan attacks on U.S. troops across the border in Afghanistan. For now, it appears to have broad public support in Pakistan as well.

Army spokesman Maj. Gen. Athar Abbas said 17 suspected militants had been killed in the past 24 hours of the operation in the valley.

He said another major town in the valley, Matta, had been cleared of militants. But some 1,500 to 2,000 insurgents remain in the valley — generally hard-core fighters, Abbas said.

Some 10,000 to 20,000 civilians were in Mingora, Abbas said, and troops were trying hard to avoid striking the innocent. Mingora, which normally has at least 375,000 residents, is a major commercial center for the valley.

"The terrorists are going to use them as human shields. They are going to make them hostage, so we are moving very carefully," Abbas said. "The pace of the operation will be painfully slow. So keep patient. But the operation has started and, God willing, we are going to take it to the logical conclusion."

The military says around 1,100 suspected insurgents have died so far in the month-old offensive. It has not given any tally of civilian deaths, and it's unclear how it is separating regular citizens killed from militants. Residents fleeing the region have reported dozens of ordinary Pakistanis killed in the fight.

Abbas also said no civilians were killed during the operation in Matta. Information provided by the military and civilians is nearly impossible to verify independently because of limited access to the region.

The offensive, which covers Swat as well as some surrounding districts, also has triggered an exodus of nearly 1.9 million refugees, more than 160,000 to relief camps. Some fear the generally broad public support for the military campaign could drain away if the refugees' plight worsens or if the army gets bogged down too long.

Pakistani Prime Minister Yousuf Raza Gilani on Saturday downplayed the chances the army would expand the offensive to the lawless, semiautonomous tribal regions bordering Afghanistan where Al Qaeda and Taliban fighters have long had strongholds.

Reports that Pakistani President Asif Ali Zardari said such an expansion was in the works have already led some families to leave the South Waziristan tribal area, the main base of Pakistani Taliban chief Baitullah Mehsud.

"It is not like this," Gilani said in response to a reporter's question about a possible new front in the offensive. "We are not foolish to do it everywhere."

It would be difficult for the army to open a new front in another territory before clearing the Taliban from Swat.

Pakistan's army has long been more structured around fighting a conventional battle against rival India on the plains of the Punjab region using tanks and artillery. It has limited experience battling guerrillas in urban settings. Many Taliban fighters can simply blend into the population or melt away to the hillsides.

The army said it had made gains toward retaking another Taliban hide-out in Swat, the Piochar area, putting the militants there on the run. In a statement, it said local residents had told them they were subjected to forced labor by militants.