A woman carries a child next to collapsed houses in Bhaktapur, on the outskirts of Kathmandu, on April 27, 2015
Humanitarian agencies said Monday they were preparing a large-scale aid operation to earthquake-ravaged Nepal, with more relief planes arriving in the coming hours.
"This will be a... massive operation," Elisabeth Byrs, spokeswoman for the UN's World Food Programme, told AFP.
Officials say more than 4,000 people are now known to have died, the overwhelming majority in Nepal -- making it the quake-prone Himalayan nation's deadliest disaster in more than 80 years.
WFP experts arrived in Kathmandu on Sunday to evaluate the situation, and the agency estimates shelter and medical equipment should be the first priority.
The World Health Organization said Monday it had already distributed medical supplies to cover the health needs of 80,000 people for three months in the country.
"An additional five emergency health kits are being flown in along with surgical kits and trauma bags to meet the immediate health needs. There is an urgent need to replenish medical stocks to support the emergency response efforts," said Poonam Khetrapal, WHO's regional director for Southeast Asia.
But with food also expected to quickly run scarce, WFP has "mobilised all of our food stocks in the region," Byrs said.
WFP is loading a plane with rations of high energy biscuits in Dubai, and Byrs said it would arrive in Nepal Tuesday.
They would be distributed to survivors in the country, taken by truck where possible, but due to the massive destruction, "the relief cargo may need to be airlifted," she added.
The UN refugee agency meanwhile said it was on Monday sending nearly 20,000 plastic sheets and some 8,000 solar lamps.
About half the stocks were already in place in Nepal and the rest were being flown from Dubai to Kathmandu on Monday afternoon on a cargo plane donated by the ruler of the United Arab Emirates, UNHCR said.
WFP experts are meanwhile poring over satellite images to estimate how many people have been affected by the disaster, Byrs said.
She said the worst-hit area was in "an agricultural zone that is home to between two and three million people."
- Millions affected -
WFP has been working in Nepal since 1964, and the agency had before the quake already been planning to assist some 500,000 people who do not have enough to eat.
The International Federation of Red Cross and Red Crescent Societies (IFRC) launched an emergency appeal for 33.4 million Swiss francs (32 million euros, $35 million) to cope with the crisis and cater for around 75,000 people over the next 18 months.
But it said the number of people affected was far higher with between 4.6 and 6.6 million people thought to be living within a 100-kilometre radius of the epicentre.
"We already had 19,000 prepositioned kits" in Nepal, IFRC chief Elhadj As Sy told reporters in Geneva, adding that they included non-food items like kitchen sets and tarpaulin sheets.
"Charter flights have also been organised with relief items. We hope to get them landed as soon as possible," he said.
Simon Eccleshall, who heads disaster and crisis management at IFRC, said the "next 72 hours is critical" for people trapped under rubble, as rains could flood the debris.
The UN children's agency has meanwhile warned that the quake had left nearly one million youngsters in desperate need of assistance.
UNICEF cautioned that the thousands of children camping out in the open in the capital Kathmandu were particularly at risk of disease.
UNICEF said it was mobilising staff and sending two cargo flights with 120 tonnes of humanitarian supplies, including medical and hospital supplies, tents and blankets.