KOTA, Pakistan - Helicopter-borne Pakistani soldiers swooped into a Taliban stronghold in a remote corner of Swat on Tuesday as the United Nations urged help for hundreds of thousands of people displaced by the fighting.

The military's offensive in Swat, 130 km (80 miles) northwest of Islamabad, is seen as a test of the government's commitment to face up to a growing Taliban insurgency and comes after the United States accused it of abdicating to the militants.

The fighting has caused a civilian exodus from the valley, once a tourist destination, raising fears of a humanitarian crisis.

A senior military official overseeing help for the displaced said an estimated 800,000 civilians had fled from the latest fighting. They were joining about 500,000 displaced by earlier fighting in the northwest, said Brigadier Aamir Raza Qureshi.

On Tuesday, helicopters flew soldiers into the Peochar valley, a side valley running northwest off the main Swat valley, where the Taliban have a headquarters, the military said.

Their mission is to conduct search and destroy operations, military spokesman Major-General Athar Abbas told a briefing. The militants are on the run.

Residents said troops had also been seen moving on the ground toward Peochar.

The offensive was launched last week when President Asif Ali Zardari, the widower of former Prime Minister Benazir Bhutto, was in Washington assuring a nervous United States his government was not about to collapse and was committed to fighting militancy.

A February pact aimed at ending violence in Swat, which effectively handed the militants control, had raised fears of a gradual Taliban takeover of more areas in the nuclear-armed country, which is vital to U.S. efforts to defeat al Qaeda and stabilize Afghanistan.

Military spokesman Abbas said 751 militants had been killed in the offensive while 29 soldiers had been killed and 77 wounded. Most reporters have left Swat and there was no independent confirmation of that estimate of militant casualties.

Taliban spokesmen were not available for comment.

There were also clashes in Imam Dehri, the home town of Fazullah, the Taliban chief in Swat. Four militants were killed there, Abbas said.

In all, about 15,000 members of the security forces are facing about 5,000 militants in the region, the military says.

The United Nations has warned of a protracted humanitarian crisis for a country already being propped up by a $7.6 billion International Monetary Fund loan.

The U.N. refugee agency has opened stockpiles of supplies to help the displaced and is airlifting in tons more supplies.

The speed and scale of this crisis is posing huge challenges for the government and the humanitarian community, U.N. High Commissioner for Refugees spokesman Ron Redmond told a news briefing in Geneva.

Richard Holbrooke, U.S. special envoy for Afghanistan and Pakistan, told a Senate panel in Washington, We have a major, major refugee crisis and the State Department was discussing adding to the more than $57 million the United States had provided.

Since our national security interests are so at stake and we look like we're heading for about 1 million to 1.3 million refugees, we should not ignore that, he added.

People in northwestern Pakistan had for decades been generous to millions of Afghan refugees driven from their homes by war.

Now that they themselves are uprooted, they deserve international help, Redmond said.

The World Food Program said it had enough wheat, pulses, and other staples to feed 1.5 million people for three months, which may not be enough to deal with the crisis.

The World Health Organization warned that the displaced faced serious risks of disease outbreaks and malnutrition.

Prime Minister Yusuf Raza Gilani told the National Assembly

on Monday that the government would soon organize a conference of aid donors to marshal funds.

Most political parties and many members of the public support the offensive, but that could change if civilians displaced in what the government say is the country's largest-ever internal migration are seen to be suffering or if many are killed.

Stock market investors have been unnerved by the fighting and political tension in the commercial hub of Karachi but the main index rose 2.44 percent on some aggressive buying by government institutions, dealers said.

Separately, suspected U.S. drone aircraft fired missiles in a Pakistani region on the Afghan border, killing at least eight people, military and intelligence officials said.

The attack took place in a mountainous region of South Waziristan, a known al Qaeda and Taliban hotbed, the officials said.