Catastrophic flash floods triggered by a glacial collapse in the Himalayas have killed more than 1,000 people in Nepal and China, with thousands still missing.

Nepal has reported nearly 1,000 deaths and thousands of people unaccounted for, while China has confirmed additional deaths and missing people in Tibet.

Rescue efforts have been complicated by destroyed roads, bridges and other infrastructure, while hundreds of hydropower workers may still be trapped inside flooded tunnels.

More than 11,800 people have been rescued, with helicopters, international rescue teams and search dogs helping reach isolated communities, the AP has reported.

Authorities have begun mass burials of unidentified victims as Nepal continues recovery efforts and warns that climate change may be increasing the risk of similar disasters in the Himalayan region.