Thousands of people flocked to Lao Cai Province's Sapa town, jamming highways, to enjoy the snow as a cold front swept down from Siberia and northern China, with temperatures dropping to decades ago lows.

Traffic on National Highway 4D was at a standstill for 10 hours on January 24.

Colonel Nguyen Thien Tuan, of Lao Cai Province's Traffic Police Board, said officers had been sent to untangle traffic jams, but kilometres-long tailbacks had made the task difficult. Trucks have been banned from Lai Chau Province to O Quy Ho Pass.

Trains were unaffected, with passengers reporting packed carriages but no delays.

Freezing temperatures were reported across the northern regions -- minus 0.4C in Tam Dao (Phu Tho Province), minus 0.2C in Dong Van (Ha Giang Province) and minus 4C in Mau Son Mountain (Lang Son Province).

In Hanoi, the temperature dropped below 5C, with snow reported at Ba Vi District, about 50km from the capital.

The cold snap is expected to last until January 27.

Photos about the traffic jam on National Highway 4D: