Restoring Nepal's world heritage sites to their former glory could take years of work and cost millions of dollars, the head of the country's Archaeology Department said on Monday (4 May).

Large parts of Nepal's Unesco world heritage sites which attract thousands of tourists each year were destroyed when the 25 April earthquake struck.

"It takes time, I think, Nepalese billion currency. (Billions?). Billions, you understand? Exactly, I'm not ready to give you because this is, it takes time," said Bhesh Narayan Dahal, the Director-General of the Archaeology Department of Nepal.

Billions of Nepalese rupees is equivalent to millions of US dollars. He said nearly 200 heritage sites had been damaged by the quake.

"I can say (tell) you, 90% (of the) heritage (sites) are damaged. Some are partly cracked, some are totally demolished, some are, we want to demolish," he said.

Among the most famous tourist attractions that bore the brunt of the devastation from the 7.8-magnitude earthquake are the three Durbar Squares in Kathmandu, Bhaktapur and Patan.

Durbar Squares are plazas opposite old royal palaces in Nepal, consisting of ancient temples, open courts and fountains. The government has begun work to clear rubble and debris in these areas.

Dahal said archaeologists were working to reconstruct the buildings, which involves meticulous drawings of the old structures. They will use the same type of construction material that was originally used to restore them. He said the process would take years.

"I think, I promise you, five to seven years. It takes time, (but) we can rebuild again because this is necessary, this is our pride, we will build it," Dahal said.

"These are the gifts of our ancestors, we have to build it because this heritage helps us in the tourism industry," he added.

The Archaeology Department said the designing and drawing was expected to take around a year and was the most essential part of the reconstruction.

Unesco has offered support for the conservation and recovery of the sites along with other international donors who are ready to provide the funds needed for reconstruction.

Government data shows that 800,000 foreign tourists visit Nepal every year with the heritage sites, monuments and temples a popular attraction.