Casualties from the floods sweeping parts of China's Xizang Autonomous Region continue to climb. Local authorities put the toll at 31 dead and 531 missing.

Terrain explains much of the scale. Settlements strung along high mountain valleys leave little room for water to drain during concentrated downpours, so levels rise sharply within a short time. Landslides that temporarily dammed rivers before bursting are reported to have compounded the damage.

Access itself is the obstacle for rescuers. Main roads are severed in multiple sections and communications are down, so helicopter insertion and airlifted supplies are running in parallel. High altitude makes helicopter operations more demanding, a further constraint.

Authorities said temporary shelter, drinking water and medicine supply systems are in operation. With the number of displaced still unsettled, the tallies are likely to keep changing as searches proceed.

The disaster is being discussed alongside broader weather anomalies across the Himalayan region. Record rainfall was observed in neighbouring countries over the same period, and arrangements for sharing flood information across transboundary river basins have resurfaced as an issue.