Three police officers on duty were killed after their car crashed into a bus travelling on the opposite direction in northern Vietnam on Friday afternoon, authorities said.

Three passengers among dozens on the bus, travelling from Hanoi to Lang Son Province, were slightly injured.

The seven-seater police vehicle carrying the three officers from Lang Son was severely damaged.

Authorities are trying to find out what caused the crash, which happened during heavy rain at around 2:30 p.m. on Friday.