Some relief material for survivors of Nepal's devastating earthquake is being held up at the country's only international airport because of customs bottlenecks, the United Nations has said.

The government in Nepal has ruled out the possibility of finding more survivors buried in the rubble from last weekend's massive earthquake.

It has also announced that the death toll has risen to 6,621.

As well as updating the death toll, Mr Dhakal put the number of injured at 14,023.

Material is piling up at the Kathmandu airport instead of being ferried out to victims, UN Resident Representative Jamie McGoldrick said.

Nepal exempted tarpaulins and tents from import taxes yesterday.

Mr McGoldrick said the government had to loosen customs restrictions further to deal with the increasing flow of relief material.

There was no immediate response from the government.

However, Finance Minister Ram Sharan Mahat had appealed to international donors to send tents, tarpaulins and basic food supplies, saying some of the items received were of no use.

"We have received things like tuna fish and mayonnaise. What good are those things for us? We need grains, salt and sugar," he told reporters.

US military aircraft and personnel are to arrive in Kathmandu today to help in relief operations.

One of their tasks would be to deal with the growing piles of aid material.

Nepali government officials have said efforts to step up the pace of delivery of relief material to remote areas were frustrated by a shortage of supply trucks and drivers, many of whom had returned to their villages to help their families.

"Our granaries are full and we have ample food stock, but we are not able to transport supplies at a faster pace," said Shrimani Raj Khanal, a manager at the Nepal Food Corp.

Army helicopters have air-dropped instant noodles and biscuits to remote communities but people need rice and other ingredients to cook a proper meal, he said.

The 7.8-magnitude quake was the deadliest in Nepal for more than 80 years.

It devastated vast swathes of the country when it erupted around midday last Saturday and reduced much of the capital, Kathmandu, to ruins.

Teams there with sniffer dogs are moving slowly through ruined buildings to locate bodies still buried in the rubble a week after the disaster.     

Elsewhere, volunteers are stacking up bricks recovered from the debris to begin the slow process of reconstruction.

Many Nepalis have been sleeping in the open since the quake, with survivors afraid to return to their homes because of powerful aftershocks.

Tents had been pitched in Kathmandu's main sports stadium and on its only golf course.

According to the United Nations, 600,000 houses have been destroyed or damaged.

It said eight million of Nepal's 28m people were affected, with at least 2m needing tents, water, food and medicines over the next three months.

The worry now is how to prevent the outbreak of disease.

"Hospitals are overflowing, water is scarce, bodies are still buried under the rubble and people are still sleeping in the open," Rownak Khan, UNICEF's deputy representative in Nepal, said in a statement.

"This is a perfect breeding ground for diseases."