The August 26 floods on the China-Nepal border, triggered by a glacial mountain collapse killed more than 1,300 people and left more than 5,600 missing in both countries.

"Time is against them," the UN office in Nepal said.

"For those who survived, the flood has taken almost everything that made daily life possible," it added.

The floods tore through mountain districts, destroying roads, bridges and hydropower dams.

"UN Nepal and partners are appealing for nearly $50 million to deliver cash, food, shelter, clean water, healthcare and other life-saving assistance to the most devastated communities," it said.

"Health authorities are already watching fever and diarrhoea in displacement centres," the UN appeal added.

"Human remains and thousands of dead livestock still lie in the debris, raising the risk of communicable disease outbreaks."

Communities near the epicentre remain cut off.

"Families whose homes are gone need durable shelter in weeks, not months," the appeal said.

A UN preliminary assessment estimated building damage from Nepal's floods at $158 million alone, warning that total losses -- including damage to roads and bridges, agricultural land and humanitarian costs -- would be far higher.

Nepal has suggested the reconstruction bill could run into "billions" of dollars.