ISLAMABAD, Pakistan - Islamist extremists who flowed out of Afghanistan fought a second day of fierce battles with Pakistani security forces Thursday in one of the deadliest clashes on the Pakistan side of the frontier in months. Authorities said 63 people were dead.

Signaling a deepening of the rift with the United States and voicing Islamabad's anger over the attacks, the government issued a statement late Thursday voicing Pakistan's "strong concern" about the attack.

Pakistan's military had initially said the assault was the work of about 200 extremists, but the government statement put the number at 300 to 400. It said the fighters "attacked villages and burned schools."

The extremists' attack and Pakistan's reaction contradicted the U.S. narrative about the poorly defined and porous border. Typically, extremist cross-frontier movements originate in Pakistan, leaving the United States and NATO to gripe at Islamabad over its failure to stop the infiltration.

The new battles found Pakistan the aggrieved party, lending credence to Pakistani army commanders' complaints that NATO was failing to crack down on extremists sheltering on the Afghan side of the rugged frontier.

The government statement said the foreign secretary had "stressed the need for stern action by the Afghan Army, US and NATO/ISAF forces in the area against militants and their hideouts in Afghanistan and against organizational support for the militants."

The fight started when the extremists crossed into Pakistan on Wednesday. By nightfall Thursday, 25 soldiers, 35 attackers, and three civilians had died in fighting, according to the regional police chief, Ghulam Mohammed.

Beyond emphasizing the difficulties of fighting an enemy that pays no attention to borders, the battle pointed to possible trouble for both the United States and Pakistan when Washington begins withdrawing troops later this year.

Pakistan is already complaining that NATO doesn't have enough troops along the Afghan side of the border.

In the past, NATO and Pakistani forces staged coordinated "hammer and anvil" operations on the border, but relations between Washington and Islamabad hit a particularly rough patch, especially since the unilateral American raid that killed Osama bin Laden on May 2.

Even so, NATO officials say that border cooperation has not suffered as a result of the chill in ties.

Pakistan's northwestern border with Afghanistan is home to thousands of local and international al-Qaeda and Taliban extremists. In broad alliance, they focus attacks on international and Afghan forces across the border, the Pakistani state, or spend time plotting and training for international extremist attacks.

Under heavy American pressure, Pakistan's army has moved forcefully into parts of the mountainous and sparsely populated region over the last four years. It previously had little or no presence, but the extremists have proved resilient.

Petraeus on Civilian Deaths

Gen. David Petraeus, the top NATO commander in Afghanistan, said Thursday that he was committed to reducing the loss of innocent lives to an absolute minimum.

The statement marked Petraeus' latest attempt to ease President Hamid Karzai's anger over civilian casualties. Karzai exploded in rage after a recent air attack that killed at least nine civilians in Helmand province in

the south.

After that attack, Karzai ordered the U.S.-led coalition to stop bombing homes because too many civilians were being killed.

It was Karzai's strongest-ever statement against NATO alliance air strikes and further complicated a difficult relationship with the Obama administration as it prepares a troop drawdown.

In a visit to Khost province in eastern Afghanistan along the Pakistan border, Petraeus expressed his "absolute commitment to reducing to the absolute minimum the loss of innocent, civilian lives" during operations targeting insurgents.

NATO has significantly reduced civilian casualties in recent years, but civilians deaths from insurgent attacks have spiked. Petraeus and Karzai are expected to discuss

the sensitive issue this weekend.

- Associated Press