The United Nations resumed crucial relief flights to earthquake-devastated areas of Pakistan on Wednesday, but the race to save hungry and freezing victims was stymied by new landslides.

Helicopters flew again after being grounded for three days by the harsh Himalayan winter. But key roads to quake-wracked areas have been blocked by landslides and avalanches triggered by heavy rain and snow.

"A lot of roads are being cut off by landslides," said U.N. World Food Program spokeswoman Caroline Chaumont.

The agency has organized more than 9,000 flights since the Oct. 8 earthquake killed 87,000 people and left 3.5 million homeless.

Overland routes bring food and supplies to roughly 500,000 of the 1 million people being fed by the WFP. Helicopters, by contrast, reach only 380,000.

Relief workers worry that the harsh winter weather will intensify hunger and misery in the quake-wracked region of Kashmir and add to the death toll. Some supplies were delivered during brief breaks in the snowfall.

Main roads leading to the Jhelum and Neelum valleys in Kashmir, where thousands of refugees await, are blocked at several places, said Maj. Farooq Nasir, the Pakistani military relief commander for the region.

Bulldozers are trying to clear the roads, but authorities said Monday it would take days to re-establish the route after the rain and snow stops.

Pakistan's Meteorological Department is forecasting scattered rain and snowfall for the next two or three days.

Aid flights have been suspended twice in the past three weeks. On Wednesday, the United Nations was operating seven helicopters, including two CH-47 Chinooks, said the WFP's Bogdan Karakoy.

"The operation is going to full power," Karakoy said. "We will pray for good weather."

Two people were airlifted out of the quake zone Wednesday to seek medical attention, said Red Cross official Rashad Akhundov. A woman required treatment for a broken leg, while a 3-month-old boy suffered hot-water burns.

Former President George H.W. Bush, the special U.N. envoy for the South Asian quake, was forced to cancel a trip to the quake zone Tuesday because of bad weather. Instead, he toured a tent camp for about 30,000 survivors in Islamabad.

Bush said he will work to encourage international donors to follow through on aid pledges. Despite frequent calls for donations, officials say the U.N. is still far short of the $550 million it needs to keep its projects running at full capacity.

International aid organizations are pleading for more money from rich nations and corporations to help stave off deaths that could result from frigid temperatures at higher elevations in Kashmir. Many are without adequate shelter, clothing and are heavily reliant on food aid.

Also, a group of medics from NATO closed a field hospital and left the country.

About 100 doctors and paramedics from NATO's main Military Relief Hospital left early Tuesday, said alliance spokesman Maj. Eric Bruijn. NATO deployed 1,000 medics, engineers and pilots to help in the relief efforts but had said it planned to leave by Feb. 1.