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UN and private aid workers said Thursday that the pressing need to shelter as many as 3 million Pakistani earthquake survivors before the harsh Himalayan winter sets in is threatening to become one of the greatest human disasters the world has ever faced.

Compounding the problems posed by the sheer number of people displaced–three times as many as those affected by the Indian Ocean tsunami last December–are the mountainous terrain and the onset of a winter that is likely to arrive in less than three weeks and sever the stricken mountain hamlets of the north from the rest of the country until spring.

And yet, whether out of fatigue after a year of seemingly endless natural disasters or because the quake struck in Pakistan, aid officials say, the international response has been weak. Even in the face of the epic destruction, foreign donors have so far pledged less than $90 million, or barely a quarter of the $312 million that the United Nations estimates it will need for immediate relief.

“It’s the most difficult humanitarian crisis ever,” said Andrew Macleod, chief operations office in the UN Emergency Coordination Center in Islamabad, “because the scale is huge, the logistics are so difficult and there’s such a brutal winter coming on.” In recent days, as his office assessed the damage across the far-flung hamlets dotting the Himalayas, the most credible estimates turned out to be “the worst-case scenarios,” Macleod said.

“We have never seen anything like this,” he added. The quake struck an isolated, mountainous area of about 11,000 square miles, roughly the size of Maryland.

Of the few roads that cross the hills and valleys, many have been cut off, and the aftershocks start landslides that block them all over again. The people who live in the hills are unable or unwilling to leave their land and go down to the lowlands, where aid is available.

Even with helicopters from abroad, there were fewer than 80 working helicopters in the country as of Thursday, 12 days after the earthquake. UN officials say they need many more. In perhaps the starkest statement of need, there are not enough cold weather tents in the world to house what the International Organization for Migration estimates to be as many as 550,000 families left homeless.

The snow will come before enough can be made. How many people will have to survive the winter without shelter is unclear. UN officials could estimate only that it would be in the tens of thousands.

“I think there is a requirement for the world community to realize the enormity of the challenge we are facing because of the nature of the terrain and the devastation,” said Pakistan’s disaster response chief, Maj. Gen. Farooq Javed.

Foreign donors have so far pledged $86 million of the $312 million the UN says is needed for relief in Pakistan. By contrast, foreign donors gave $43 million to stave off starvation in Niger, more than half the $81 million urged by the UN. And the emergency appeal for aid in the December tsunami produced 80 percent of what was needed within 10 days.

“This is not enough,” the UN’s top relief envoy, Jan Egeland, said in Geneva on Thursday. “We have never had this kind of logistical nightmare, ever. We thought that the tsunami was as bad as it could get. This is worse.”