KATHMANDU (AFP) - Relief workers face huge challenges in reaching the remote mountain areas worst hit by the earthquake in Nepal, with heavy rains exacerbating the problems, the UN said on Tuesday.

Resident coordinator for Nepal Jamie McGoldrick said around half those living in the districts worst affected by the earthquake have lost their homes, describing the suffering as "horrendous".

"Our big challenge now is getting out of Kathmandu and into the more affected areas, the epicentre," he said.

"We know the major roads are open, but take those roads to the smaller villages - the very high altitude, rough terrain, and the challenges of getting to these areas is significant."

McGoldrick said he had witnessed "horrendous" suffering after the quake, which killed at least 5,057 in Nepal while more than 100 people died in neighbouring countries.

Around 8,000 people had been injured while the United Nations estimated that eight million people had been affected.

Heavy rain and regular aftershocks were exacerbating the stress, he said, adding that "72 hours later people are still in a hardship mode".

"Unfortunately the weather will exacerbate the already terrible situation that is there," he said, warning that the upcoming monsoon rains would worsen the situation of people left homeless still further.

The UN said earlier that eight million people had been affected by the 7.8-magnitude quake that hit on Saturday.

More than 1.4 million need food, while water and shelter are also in short supply, it said in a report.

McGoldrick said he had seen doctors in a hospital outside Kathmandu operating on seriously injured patients during an aftershock, as lights flashed.

Jagdish Chandra Pokherel, a Nepal Army spokesman, told AFP: " The terrain is such that very remote areas take a very long time to reach and without being there physically we won't be able to reach them, help them, rescue them. Our troops are trying their best."

Rain had also hampered the effort, Nepal police spokesman Kamal Singh Bam said.

With fears rising of food and water shortages, Nepalis were rushing to stores and petrol stations to stock up on essential supplies in the capital Kathmandu.

Nepal has declared a state of emergency after the disaster, its deadliest in more than 80 years.