As Pakistan's military continues to battle Taliban militants in Swat valley, a suspected U.S. drone fired on a compound in South Waziristan, killing up to eight militants. Aid groups are expressing concern about the estimated 500,000 people who have fled the military's offensive against the Taliban.

Pakistan's army says its forces killed another 51 militants in the last day, bringing the total death count to 751 Taliban fighters. The army says 29 soldiers have died and 77 were wounded during the same period.

The army has not provided evidence of the high number of militants killed, nor have officials attempted to project how many civilians have died in the operation. Two previous military offensives in Swat failed largely because civilian casualties eroded local support for military intervention.

Army Spokesman Major General Athar Abbas told reporters in Islamabad that so far, the military has focused on using heavy firepower in more isolated areas to try to reduce civilian casualties.

"It has not gone, so far, to the hardcore urban fight," he said. "Until we get into that we expect that we will be able to take out most of the civilians. Because it would be unwise to go into an urban area against the militants where the civilians are."

Abbas says this is why the military continues to lift curfews a few hours a day in some places to allow civilians to flee before troops take more aggressive action. The army estimates more than 100,000 civilians are still trapped in the conflict zone, but each time the curfew is lifted the Taliban take advantage of the lull in fighting.

"The miscreants take the best advantage of the relaxed curfew hours and move around and either escape or enhance their positions or strengthen the area," said Abbas. "We are mindful of that and we are taking all measures."

The United Nations says more than 500,000 people fleeing the fighting have been registered by authorities. The vast majority of them are living with friends, relatives and in temporary camps.

Their arrival has raised the total number of displaced people in the northwest to about one million people. U.N. officials say caring for the civilians is posing huge challenges for the government and aid groups.

Unlike previous military interventions in Swat, the recent offensive has broad public and political support. U.S. officials also have praised the action following months of criticism over the attempted peace deal with the militants.

Pakistani security analyst Mahmood Shah says the offensive has helped bridge the gap in trust between Pakistan and the United States. But he says drone strikes, such as the one Tuesday in South Waziristan, show that significant differences remain.

In recent weeks American news media have reported that U.S. officials continue to believe the tactic is highly effective at killing top al-Qaida leaders in places where they are largely beyond the reach of the Pakistani government.

Mahmood Shah says although some militant leaders have been killed, they have not been worth the damage to public opinion.

"It hasn't helped. The elimination of a couple of people, it could be 8 or 10 so far," he said. "But the amount of damage and destabilization in the country that we are starting to see - this is what the Americans do not understand."

Despite Pakistan's public objections, the United States has sharply increased the number of drone strikes in the last year, with more than 40 since the beginning of 2008. The strikes have increased in frequency in recent months.