MINGORA, Pakistan, (AP) – Taliban militants blocked roads with rocks and trees, preventing terrified civilians from fleeing a Pakistani valley Thursday as the army stepped up a ground and air assault on the guerrillas that has been applauded by the U.S., witnesses said.

Destabilizing violence is flaring in Pakistan just as its embattled president is appealing in Washington for more help to reverse the extension of Taliban-held territory to within 60 miles (100 kilometers) of the capital. The U.S. is particularly concerned by the unrest because its troops are fighting an increasingly virulent insurgency in Afghanistan fed from militant havens in Pakistan’s lawless border area.

Officials are bracing for a mass exodus from the Swat Valley, a former tourist destination where fighting has resumed after the breakdown of a controversial peace deal earlier this week. The military claimed to have killed more than 80 militants in the region on Wednesday. There has been no official word on civilian casualties.

More than 500,000 Pakistanis driven out by fighting in other regions of the northwest are already living in makeshift camps or with relatives, adding a growing humanitarian crisis to the country’s daunting security, economic and political problems.

With Taliban militants roaming the streets of Mingora, Swat’s main town, on Thursday and troops launching artillery and airstrikes on militant targets from helicopter, many residents hunkered down in their homes.

The army announced it was relaxing its blanket curfew in the area, but some of those who tried to make a swift exit said militants blocked their way.

Ayaz Khan, a 39-year-old from the Kanju area of Swat, said he loaded his family into his car early Thursday but that rocks, boulders and tree trunks has been laid across the roads, forcing him to turn back.

“I am helpless, frustrated and worried for my family,” he told an Associated Press reporter by telephone from his home. He appealed to authorities to clear the barriers and let people move to safety.

A health worker living in Mingora said militants had warned her to stay in her home.

“During the whole of last night, I heard firing, and again this morning,” said the woman, who would only give her first name, Maryam, for fear she could be targeted for speaking with a reporter.

“I don’t know when some weapon will hit our home and kill us,” she said.

Washington has said it wants to see a sustained operation in Swat and surrounding districts, mindful of earlier, inconclusive offensives elsewhere in the Afghan border region. Eight years after the Sept. 11, 2001, attacks, the area remains a haven for al-Qaida and Taliban fighters blamed for spiraling violence in Pakistan and Afghanistan.

But uprooting the insurgents from the valley will mean civilian casualties, property damage and massive disruption which could sap the resolve of the government, which is struggling to convince the nuclear-armed Muslim nation that fighting the militants is in its interests as well as those of the U.S.

President Barack Obama and Pakistani President Asif Ali Zardari met Wednesday in Washington to explore ways to boost the country’s antiterror fight, seen by many as the most pressing foreign policy issue facing the U.S. administration.

“Pakistan’s democracy will deliver,” Zardari said in Washington.

U.S. Secretary of State Hillary Rodham Clinton said the military offensive against the Taliban was a positive sign.

“I’m actually quite impressed by the actions the Pakistani government is now taking,” she said. “I think that action was called for, and action has been forthcoming.”

The Swat accord began unraveling last month when Taliban fighters moved from the valley into the nearby district of Buner, even closer to Islamabad, prompting an operation that the military says has killed more than 150 militants but has yet to drive them out.

The Swat Taliban are estimated to have up to 7,000 fighters — many with training and battle experience — equipped with rocket-propelled grenades, explosives and automatic weapons. They are up against some 15,000 troops who until recent days had been confined to their barracks under the peace deal.