Assessment teams say they have found survivors in a “desperate situation”.
Nepal says 6,204 people are known to have died in the 7.8-magnitude quake and 13,932 were injured.
But the fate of thousands more in many remote areas remains unknown and the government has warned that the death toll could rise to more than 10,000.
Although rescue teams from Nepal and the international community are operating in the capital Kathmandu and the surrounding area, landslides and poor weather have hampered efforts to reach isolated districts.
The home ministry says that 20 helicopters carrying relief have landed in the districts of Sindhupalchok, Dolakha, Rasuwa, Nuwakot, Dhading and Gorkha.
But correspondents say that there is a severe shortage of helicopters, and although China is expected to send more, Nepal has appealed to other countries for further aircraft.
The United Nations on Wednesday launched a “flash appeal” for $415m (£270m) to assist Nepal over the next three months – but on Friday it told the BBC it had so far received only $5.8m – 1.4% – in confirmed funding.