MINGORA, Pakistan - Pakistani aircraft pounded Taliban positions in the Swat valley on Thursday as thousands of people took advantage of a break in a curfew to flee from the region.

The government's handling of Swat has become a test of its resolve to fight a growing Taliban militant insurgency. President Asif Ali Zardari assured U.S. President Barack Obama in Washington on Wednesday of Islamabad's commitment to defeating al Qaeda and its allies.

A February peace pact aimed at ending Taliban violence in Swat has collapsed and on Wednesday the military launched assaults in the outskirts of the region's main town of Mingora.

We can't stay here when bombs are falling all around, said resident Mohammad Hayat Khan as he loaded his family of 14 onto a pick-up truck. He said there had been shelling near his home.

Many other people were heading out of Mingora on foot, loaded up with whatever they could carry.

About 40,000 people have been displaced from Swat and nearby areas since fighting flared last month, provincial authorities said, adding that up to 800,000 people could flee from the valley, which has a population of about 1.6 million.

Several hundred thousand had already fled fighting in different parts of the northwest since August.

The United Nations High Commissioner for Refugees expressed his deep concern about the safety of tens of thousands of displaced while the International Committee of the Red Cross said a humanitarian crisis was intensifying.

Authorities agreed in February to a Taliban demand for the introduction of Islamic sharia law in the former tourist valley but the militants refused to disarm, and pushed out of Swat into neighboring districts.

The aggression raised alarm in the United States and led to accusations the government was capitulating to the militants.

Security forces launched an offensive on April 26 to expel militants from two of Swat's neighboring districts, Dir and Buner, and security has deteriorated sharply in Swat since then.

Army chief General Ashfaq Kayani told his top commanders the army was fully aware of the gravity of the internal threat and would employ requisite resources to ensure a decisive ascendancy over the militants, the military said.

Investors in Pakistani stocks have been unnerved by the fighting and the main index ended 1.02 percent, or 73.20 points, lower at 7,125.66 points on Thursday.

CLERIC'S SON KILLED

Zardari met Obama and Afghanistan's President Hamid Karzai in Washington on Wednesday. Obama later said both men fully appreciate the seriousness of the threat posed by al Qaeda and their allies.

The road ahead will be difficult, Obama said, with Karzai and Zardari at his side at the White House.

But let me be clear -- the United States has made a lasting commitment to defeat al Qaeda but also to support the democratically elected sovereign governments of both Pakistan and Afghanistan.

Pakistani security forces said at least 64 militants were killed in Swat and the nearby Buner district in fighting on Wednesday. Two soldiers were also killed.

A Taliban spokesman said more than 30 civilians had been killed in Swat.

There was no independent confirmation of either side's tolls.

Security forces used jets and helicopters to attack militant strongholds in at least four parts of the Swat valley on Thursday, residents and government officials said.

There was no word on casualties but a son of radical cleric Sufi Mohammad, who brokered the Swat deal, was killed in a clash between soldiers and militants in neighboring Dir district, the military said, adding nine other militants were killed.

People began streaming out of Mingora when authorities temporarily lifted a curfew. Swat's top administrator, Khushal Khan, said people were not being advised to leave but authorities were helping those who wanted to go.

There are troop convoys coming and once that is over, public transport will be able come into the town. In the meantime, we are extending whatever help possible to people leaving, he said.

Many displaced stay with friends, relatives and in rented accommodation but the exodus puts an extra burden on an economy propped up by a $7.6 billion International Monetary Fund loan.

Authorities have set up camps for people leaving Swat with the help of the United Nations and aid agencies.