The BBC's Jon Donnison reports from a street
destroyed by "a wall of water"
At least 10,000 people are feared to have been killed by Typhoon Haiyan, which struck the central Philippines on Friday.
The UN says more than 11 million people are believed to have been affected by the storm and some 673,000 displaced.
Several countries have deployed ships to help the relief effort, but bad weather is hampering aid distribution.
Valerie Amos, the UN's Under-Secretary-General for Humanitarian Affairs and Emergency Relief, has arrived in Manila to head the aid operation.
She told the BBC the storm had been far worse than expected, and that people in the affected regions were "absolutely desperate".
Valerie Amos told the BBC she hoped to visit the affected region in person
"They need food, they need water, they need shelter. People need to be protected," she said.
Helicopters were also in urgent need to help with assessing the damage, she said.
Baroness Amos told reporters the UN would work alongside the Philippine government, and that efforts would focus on "food, health, sanitation, shelter, debris removal and also protection of the most vulnerable".
"I very much hope our donors will be generous," she said.
The BBC's Imogen Foulkes, at the UN in Geneva, says the operation will be one of the most challenging the body has ever mounted.
Survivors are scattered across several of the 7,000 islands which make up the Philippines, says our correspondent, and many areas have not yet been reached, so the figures of those in need - and the estimated death toll of 10,000 - could rise.