Towns and villages near the epicentre of Saturday's earthquake that claimed at least 6,200 lives in Nepal have suffered "almost total devastation", the Red Cross says.
The International Federation of Red Cross and Red Crescent Societies (IFRC) said on Thursday that they were "extremely concerned" about the welfare of hundreds of thousands of people, with those in remote areas outside the Kathmandu Valley facing an unknown fate.
The devastation appeared particularly dire in the Sindupalchowk region, a mountainous area northeast of Kathmandu, which was becoming a major focus of international relief efforts with up to 90 percent of homes destroyed in some parts.
"Six Red Cross assessment teams are reporting that some towns and villages in the worst-affected districts close to the epicentre have suffered almost total devastation," it said in a statement.
Launching an appeal for $415m in aid, the UN said it would take a marathon effort to help the people of one of Asia's poorest countries.
The International Monetary Fund said it was ready to extend aid to Nepal and would send a team to assess the situation "as soon as possible".
Nepalese and Indian military helicopters have also been flying in aid to remote areas of Gorkha district, another badly-hit region, which otherwise takes up to 12 days to reach by foot, the WHO said.
The gloomy assessments of the situation came as the rescue of two survivors brought some hope to people in the capital.
The rescue of 15-year-old Pemba Tamang, who told AFP news agency he stayed alive by eating a jar of ghee (clarified butter), was hailed as a miracle five days after the massive quake.
Crowds of bystanders massed to watch the drama unfold at a guesthouse in the capital of Kathmandu, greeting the teenager with cheers as he emerged from the ruins of the building.
Caked in dust, Pemba was fitted with a neck brace and hooked up to an intravenous drip before being lifted onto a stretcher and then raced to a field hospital where he was found to have only minor cuts and bruises.
"I never thought I would make it out alive," the teenager said at the Israeli military-run facility where he was being kept for observation.
Pemba, who worked at the guesthouse as a bellboy, said he had been eating lunch next to the reception when the ground started shaking.
"I tried to run but... something fell on my head and I lost consciousness - I've no idea for how long," he said.
"When I came round, I was trapped under the debris and there was total darkness," he added. "I heard other people's voices screaming out for help around me... but I felt helpless."
Al Jazeera's Mohammed Jamjoom, reporting from Kathmandu, said many people in the capital were calling the rescue a "near miracle".
"This is something that really boosts the morale of rescuers at the scene," Jamjoom said.
Just hours later, a team pulled a kitchen worker in her thirties named Krishna Devi Khadka from the rubble of another hotel just streets away, to loud appreciation from the multinational team of rescuers who had worked into the night to save her.
Emergency workers from France, Norway and Israel operating with the Nepal army and using listening devices to find survivors took 10 hours to free Khadka once they had discovered her.
"She was injured but she was conscious and talking," a Nepal army major told an AFP reporter at the scene.
The latest official toll put the number of dead at 6,204 and more than 14,000 are known to have been injured. More than 100 people were also killed in neighbouring countries such as India and China.
Source: Al Jazeera and agencies