Nepal is stepping up relief and recovery efforts following last week's devastating flash floods along the Nepal-Tibet border, while rescue teams continue searching for survivors, the United Nations said.

For thousands of affected people, the immediate priorities are receiving emergency aid, finding missing family members, recovering the bodies of loved ones and reuniting children separated from their families, a spokesperson for UN Secretary-General said.

Nepal's disaster management agency said at least 1,200 people have been killed and more than 4,200 remain missing. On the Tibetan side, at least 21 people have died and more than 540 are still unaccounted for, according to Chinese state media.

Rescue teams are also working to reach potentially hundreds of workers believed to be trapped inside hydropower tunnels in Nepal.

The disaster occurred last Wednesday when a massive flash flood, believed to have been triggered by a glacial collapse, swept through a Himalayan valley and destroyed villages in its path.

Nepal's disaster management agency said nearly 12,000 people have been rescued since the floods.

However, thousands of people remain missing, including hundreds of foreign nationals from countries such as India, the United Kingdom, Australia, the United States and Malaysia.

With a week having passed since the disaster, concerns are growing that the death toll could rise further as rescue teams reach more remote and severely affected areas.

With inputs from BBC