Authorities in Nepal warned of possible fresh flooding on Sunday, urging residents in already devastated districts to remain on high alert, as mountain rescue specialists more usually deployed to assist climbers on peaks including Everest joined the search for hydropower workers trapped in flooded tunnels.

Flood warning notices were sent to residents' phones throughout the morning after water levels rose in the Bhotekoshi river, as rescue crews worked to clear a major highway connecting Kathmandu to one of the worst-hit areas.

The combined death toll in Nepal and Tibet has risen to 750, with more than 3,000 people still missing, authorities said on Sunday.

Nepal's disaster agency put the toll in the country at 734, with 2,498 missing, while Chinese authorities reported 16 deaths and 546 missing on the Tibetan side of the border, including more than 100 Nepalis and people from over a dozen other countries.