"We are making extraordinary progress in reaching the majority of the people affected in the majority of the areas. We are also experiencing extraordinary obstacles in many, many areas," said UN Emergency Relief Coordinator Jan Egeland.

He said the estimated death toll was still about 150,000 but was likely to rise, as more bodies were found in the hardest-hit areas and survivors succumbed to disease.

The United Nations said it was concerned children orphaned or separated from their parents by the tsunami might be falling prey to criminal gangs bent on selling them into slavery.

The UN said it had received reports of adults posing as foster parents and children being shipped from Indonesia to Malaysia for sale, adding to worries about a "tsunami generation" of children also under threat of disease and hunger.

UN officials were alarmed when a colleague received a mobile phone text message that read: "Three hundred orphans aged 3-10 years from Aceh for adoption. All paperwork will be taken care of. No fee. Please state age and sex of child required."

Aid efforts in Aceh province on Indonesia's Sumatra island, where the tsunami claimed two-thirds of its victims, were hit for hours when a Boeing 737 cargo plane struck a water buffalo on a runway at the airport in the provincial capital Banda Aceh.

Only helicopters could operate from the airport until the plane was eventually dragged away. The incident highlighted the vulnerability of relief work already struggling to overcome the loss of roads and bridges swept away by the Dec. 26 tsunami.

The World Health Organisation (WHO) estimated there were more than half a million people injured and in need of medical care in six nations. Fears grew that diseases like cholera and malaria would break out among the five million displaced.

"RACE AGAINST TIME"

"It is a race against time," the WHO said in a report.

The UN agency said pneumonia, diarrhoea, malaria and skin infections had already emerged in Aceh. There were cases of gangrene among survivors with wounds exposed to polluted water. Seawater and sewage have contaminated many wells.

On the coast of Aceh, frantic women juggling babies on their hips and desperate men swarmed around a US Navy helicopter and fought over food parcels and water bottles.

"Sir, please help. Sir, please help," they shouted at a foreign reporter. "We need food and medicine."

The Aceh coastal town of Meulaboh was cut off from the world for a week after the tsunami struck and has an estimated 40,000 feared dead.

A damaged airstrip was cleared sufficiently for use by small Twin Otter aircraft, enabling medical teams to distribute bandages, dressings and painkillers.

"The casualty rates in Meulaboh defy imagination," said Aitor Lacomba, Indonesian director of aid group International Rescue Committee. "Tens of thousands need immediate assistance."

Wandering the streets was Said Taufik, 40, who lost nine family members including his wife, son and father.

His house was devastated. His furniture store was gone.

"I'd never heard the word tsunami before but I was standing near the beach and I saw the water pull away about 10 metres (yards) and I just started running," he said, weeping. "When the waves came people started shouting 'Water, water'."

In Sri Lanka, the second worst-hit country with 30,000 confirmed dead, rain again lashed eastern regions, flooding camps housing hundreds of thousands of homeless.

"Look at all this water," said Savaguru Puvaneswaran, 28, at a school housing 1,500 villagers. "The children are getting sick. There is a problem with the toilets, with all this water, with disease."