ISLAMABAD, Pakistan — Pakistani troops drove off a Taliban attack on a fort and pounded another band of militants holed up in a health center, officials said Wednesday as fighting spread to a third area of the tribal belt along the Afghan border.

As many as 49 insurgents were reported killed.

The violence came a week after the threat of impeachment forced longtime U.S. ally Pervez Musharraf to resign as president, triggering a scramble for power that resulted in the collapse of Pakistan's governing coalition.

The party led by former Prime Minister Benazir Bhutto until her assassination last December is now in a position to dominate the government, and it is toughening its stance against Islamic extremists at a time when they are becoming increasingly bold.

The Taliban have claimed responsibility for a wave of suicide bombings, including one outside the country's biggest weapons complex last week that killed at least 67 people, almost all of them civilians.

Security forces have been waging offensives against militants for several weeks in the northern Swat Valley and in the Bajur tribal area, considered a launch pad for Taliban operations into Afghanistan and a possible hideaway for Osama bin Laden.

On Wednesday, fighting spread to South Waziristan, a tribal region that has seen a stream of suspected U.S. missile attacks on al-Qaida hideouts in recent months.

The military said 75 to 100 militants assaulted the Tiarza Fort around midnight Tuesday, but troops guarding the post and a checkpoint on a nearby bridge "responded effectively and repulsed the attack."

Its statement said 11 militants were killed and up to 20 wounded, but made no mention of any casualties among the troops. Spokesmen for insurgent groups could not be contacted to discuss the claim.

Aminullah Wazir, a shopkeeper in Wana, the main town in South Waziristan, said authorities imposed a curfew in the area Wednesday. He said shops were shuttered and the streets deserted.

"We heard shelling and gunfire almost all night," Wazir told The Associated Press by telephone.

The fiercest battles in Pakistan's restive northwest have been in Swat and Bajur, where officials say hundreds of militants have been killed by military operations and some 200,000 residents have fled their homes to escape the violence.

In the deadliest incident Wednesday, troops rained gunfire and artillery shells on militants sheltering in a health center in Bajur, killing as many as 30 and wounding many more, said a military spokesman, Maj. Murad Khan.

Security forces estimated the toll with the help of intercepted radio traffic among the insurgents, he said.

Police said an additional eight militants were killed and 10 wounded when troops fired on suspect vehicles in two areas of Bajur early Wednesday.

Later in the day, militants ambushed a government convoy near Wana. Khan said there were several casualties, without giving details. But an intelligence official, speaking on condition of anonymity because he was not authorized to speak to journalists, said two paramilitary troopers were killed and several others were missing.

Pakistan's 5-month-old government initially sought to calm militant violence by holding peace talks. But the initiatives have borne little fruit, and U.S. officials have been pressing for tougher action against insurgent groups they blame for rising violence across the border in Afghanistan.

In addition to a string of suicide bombings in the last week, gunmen fired at the car of a senior U.S. diplomat in the northwestern city of Peshawar on Tuesday and a bomb killed seven people at a roadside restaurant near Islamabad, the capital.

Pakistanis and the country's Western backers worry the political turmoil since Musharraf's ouster after nine years in power is distracting the government from dealing with militants as well as taking steps to shore up the flagging economy.

On Wednesday, the Karachi stock exchange's benchmark 100-share index fell 3 percent. The index has slid more than 40 percent since April and stands at its lowest level in more than two years.

Lawmakers are to elect a new president Sept. 6. Asif Ali Zardari, Bhutto's widower and the current leader of her party, which has the biggest block of seats in parliament, is widely expected to win.

Critics have questioned Zardari's suitability in light of a Financial Times report that his lawyers told a London court that he suffered from serious mental problems.

Zardari's party said he had suffered great stress during his confinement in Pakistani jails on corruption charges, but is now fully fit to lead the country.

Pakistan's second biggest party, headed by former Prime Minister Nawaz Sharif, which bolted the governing coalition Monday, said Wednesday that a "patient" shouldn't be allowed to run for president.

"Similarly, if a sitting president suffers from such mental and psychological problems, constitutionally he cannot retain his office," said the spokesman for Sharif's party, Sadiqul Farooq.

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Associated Press writers Zarar Khan in Islamabad, Ishtiaq Mehsud in Dera Ismail Khan and Ashraf Khan in Karachi contributed to this report.