MUZAFFARABAD, Pakistan (Reuters) - The United Nations will launch a major air operation this week to ferry food and other supplies to earthquake survivors high in Pakistan's mountains in a frantic bid to beat the problems of winter.

Britain has supplied three Chinook transport helicopters that will fly up to 200 tonnes of supplies a day into the mountains for five days from Tuesday, said senior U.N. official Pat Duggan.

"It's a huge acceleration in our ability to cover the needs in these areas," Duggan told Reuters in Muzaffarabad, the ruined capital of Pakistani Kashmir, on Monday.

"The real priority is the highlands. The aim is to get their food and shelter needs in as fast as we possibly can before winter sets in and then avoid a flow of people down the hill."

The October 8 quake killed more than 73,000 people in Pakistan, most of them in Pakistani Kashmir. About 3 million people were affected and many are still in need of emergency assistance.

Cut off by landslides, the Neelum Valley -- home to around 150,000 people -- is the most pressing problem area and the focus of this week's airlift.

The United Nations and other relief agencies say communities need shelter and food supplies by the start of December, when bad weather is expected to severely hamper road and air transport.

International agencies are clamouring for funds to pay for the relief work and Pakistan is hosting a donors' conference on November 19 that U.N. Secretary-General Kofi Annan is due to attend.

On Monday, U.S. goodwill envoy Karen Hughes and Christina Rocca, assistant secretary of state for South Asian affairs, took three of America's top business leaders to Muzaffarabad to see first hand the urgent need for more aid.

Hughes was accompanied by Hank McKinnell, CEO of the world's biggest drug maker, Pfizer Inc., Anne Mulcahy, chairman of office-equipment firm Xerox Corp., and Jim Kelly, former chairman of United Parcel Service Inc.

The U.S. party, moving with a heavy security escort, went to a girls' school destroyed by the quake and visited one of the city's many tent camps before returning to Islamabad for a meeting with President Pervez Musharraf.

"People in America care very much about the girls and boys and people of Pakistan and that's why you have probably seen some of the helicopters trying to bring help," Hughes told two seven-year-old girls in a tent where classes were being held.

Doctors in Kashmir have begun a campaign to immunise 800,000 children against measles, tetanus, whooping cough, diphtheria and polio before the winter bites and people congregate in camps.

"We haven't had any fatalities yet, but still we have severely dehydrated patients coming in," said John Watson, a communicable disease expert with the World Health Organisation.

The Pakistani Ministry of Health has denied reports the sickness is cholera. But Watson said, whether cholera or not, the response to the diarrhoea outbreak had to be the same.

India and Pakistan also opened a fourth point on the border dividing Kashmir, between Tattapani on the Pakistani side and Mendhar in India, for an exchange of relief supplies, a Pakistani military source said.

The two countries agreed on November 7 to open five points on the disputed border, known as the Line of Control, but so far no Kashmiris have been allowed across.

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