Sujan Shrestha says it breaks his heart to look at the piles of rubble that are all that remain of the ancient Nepalese temples where he has worshipped all his life.

The 28-year-old shopkeeper grew up in a house just off Patan Durbar Square, a spectacular World Heritage site in the quake-hit Kathmandu Valley packed with ornately-carved Hindu temples, statues and a royal palace.

"We were always told that a big one was coming, but I never imagined that it would be this devastating," he said as he looked over the ruined square where he used to sell pashminas to tourists.

"I cannot believe that those temples collapsed, this place looks incomplete without them. It breaks my heart when I look around."

The 7.8-magnitude earthquake that hit on April 25 has devastated the Himalayan nation's rich cultural heritage and left the Kathmandu Valley's three former royal squares in ruins.

The squares, which date back to when the valley was divided into three Hindu kingdoms, are at the heart of local life as well as being a huge draw to a country that relies heavily on tourist dollars.

"The quake left an impact on 90 percent of our heritage sites. It is difficult to value the loss," said Bhesh Narayan Dahal, the director general of the Department of Archaeology.

"The very next day after the earthquake, a team was deployed to assess the damage and make an inventory of what was found.

"Our concern is to not let any antiquity or items of value to leave the site. Our teams are working to keep a record and collect details."

The UN's culture agency UNESCO said the damage to the three squares in Kathmandu, Patan and Bhaktapur -- all world heritage sites -- was "absolutely dramatic".

"We are in contact with the government and pushing them very hard to protect these sites and not to increase this damage now," said Christian Manhart, head of UNESCO in Nepal.

- 'Nature's war on us' -

In Patan Durbar Square on Friday, 500 Nepalese police officers and soldiers were combing through the rubble to try to salvage what they could for reconstruction.

"We worked all yesterday and have been working since early this morning... This is our heritage, and we have to protect it," police deputy superintendent Prakash Sharma told AFP on the square.

As soldiers stacked bricks and surveyed the scene from huge piles of rubble, local people -- many of them elderly men and women in traditional Nepalese costume -- gathered behind a rope hastily erected to keep them off the usually bustling square.

To one side, people lined up with buckets to collect water from the ornate stone taps of a traditional Nepalese step well that appeared to have survived the devastation.

"We have survived a human war, but this is nature's war on us," said Sharma, who indicated he was concerned about possible looting of priceless artefacts in the chaotic aftermath of a quake that killed thousands.

"We have stopped public access because not everyone has the right intention. We have cut off the area and been providing security to the ruins as much as possible," he said.

UNESCO's Manhart said there were detailed drawings and architectural plans, meaning the monuments could be rebuilt, as many were following the last major quake to hit the valley in 1934.

He also said the agency would work to make the centuries-old monuments more resistant to quakes in the future.

"We will deploy all our resources to reconstruct the sites within five to seven years," said Dahal of the Department of Archaeology.

"This is our pride, our identity and we will work to rebuild it."

Key facts about the Nepal quake, one week on
Kathmandu (AFP) May 2, 2015 - A week on from a 7.8-magnitude earthquake that struck Nepal, flattening large parts of the capital Kathmandu and causing devastation across the impoverished Himalayan nation, here are some key facts about the disaster.

- Death toll -

- 6,621 people are known to have died in Nepal alone and 14,023 others were injured

- More than 100 people were killed in neighbouring India and China

- 18 climbers died at Mount Everest base camp when the quake sparked an avalanche

- Two Americans, an Australian, a Japanese and a Chinese national were among the victims on the mountain

- 1,000 European Union citizens are missing, the EU says

- 12 others from EU countries have been confirmed dead

- Survivors -

- Eight million people, or around a quarter of Nepal's population, have been affected, the United Nations estimates

- 2.8 million Nepalese were displaced

- More than 3.5 million people are estimated to be in need of food assistance

- 1.7 million children are in need of humanitarian aid in the worst-hit areas, according to UNICEF

- Aid -

- Rescuers from 22 countries are involved in the relief effort while $61 million has been pledged, according to the UN

- Britain has promised $15 million

- $12.5 million has been pledged by the United States

- $15 million is being released from the UN's emergency fund

- The UN has appealed for $415 million while UNICEF is seeking to raise $50.35 million

- The United Nations' Food and Agricultural Organization has appealed for $8 million for farmers

- The World Food Programme says it needs US$116.5 million to provide food for 1.4 million people

- Reconstruction -

- 160,786 homes were destroyed and another 143,673 damaged

- The UN says that up to 90 percent of health facilities in four districts were severely damaged

- Some 16,000 schools were damaged

- Reconstruction costs could top $5 billion -- around 20 percent of the country's GDP -- according to business research consultancy IHS