There have been at least 200 cases and possibly as many as 750 at one camp for homeless quake survivors in Pakistani Kashmir, amid fears that it could be cholera, the World Health Organisation (WHO) and UNICEF said.

"Rain would be disastrous," WHO emergency coordinator Rachel Lavy told AFP at the main camp on the sports ground of the devastated university in the regional capital Muzaffarabad. About 3,000 people are living there.

"Diarrhoeal illness and rain water go hand in hand," she said.

Rain -- the first for six days -- started in quake-hit northern Pakistan and parts of Kashmir early Thursday and was due to continue on Friday, while snow is expected at night, the Pakistani meteorological department said.

Winter weather poses the biggest threat to survivors of the October 8 quake, which killed 74,000 in Pakistan and 1,300 in India. UN Secretary General Kofi Annan has warned there could be a massive second wave of deaths.

The disaster left around three million people homeless. The numbers in the camps that have sprung up in almost every town and village are swelling as people come down from the freezing Himalayan mountains.

"It's terrible. If the rain gets stronger the water will come inside and the children will get sick," said Mohammed Ramzan as he tried to fix a flimsy polythene sheet over his tent in the Muzaffarabad camp.

He has lived there with his family of seven since the quake.

Aid workers said they were now focusing on preventing life-threatening diarrhoea at the camps in Muzaffarabad, by teaching people how to keep clean, digging new latrines and setting up an isolation tent for the sick.

UN officials are still investigating whether the cases are cholera, but added that there are other waterborne microbes that could be equally serious.

"We are taking it as seriously as if it were cholera," said Jan Vandemoortele, the UN Emergency Coordinator in Pakistan. "We are still awaiting confirmation but this is in line with what we have been saying, that sanitation is a potential timebomb."

Disease is on the rise across quake-hit areas as cold weather closes in.

Doctors in the ruined northern village of Batagram said they had dozens of pneumonia cases, while the United Nations said it was investigating possible diphtheria cases in a number of towns.

Claudia Hudspeth, UNICEF's head of operations in Pakistani Kashmir, estimated that around 25 percent -- 750 people -- at the university ground camp in Muzaffarabad had been affected by diarrhoea.

"If there is rain it will escalate the situation," she told AFP. "The hygiene situation is terrible in the camp. There is open defecation, kids are playing around. It is quite a mess."

She added that there were more than 30 camps throughout Muzaffarabad, all of which could be affected.

The WHO's Lavy said there were at least 200 cases of acute watery diarrhoea at the camp in the last five days, including 55 reported on Tuesday and 77 the following day. There have been no deaths so far.

Forty other cases were recorded last week in Chinari, a small town in Pakistani Kashmir.

There was a further blow to aid efforts when a third crossing at the line dividing the Indian and Pakistani zones of Kashmir failed to open Thursday. Two others opened this week to allow relief goods across.

"The opening has been delayed by India," military spokesman Major General Shaukat Sultan said of the crossing point from Titwal town in the Indian zone to the Neelum valley in Pakistan's sector, one of the worst hit areas.

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