At least four people were injured when the brakes failed on a bus - sending it rolling backwards before crashing on a busy road.

The vehicle lost control and careened downhill in Chiang Mai, Thailand, on March 26 shortly after 10 am local time.

Footage shows the bus moments before it collided with a pickup truck injuring four passengers.

The bus driver tried to make another turn and hit a light post to slow down the vehicle before finally crashing into a barrier, bringing it to a stop.

Paramedics took four injured passengers to the Mae Taeng Hospital. They were Yoon Janta, 74, Hom Lungta, 36, Nu Jingta, 50 and Danai Janta, 13.

Bus driver Surachat Sapakepanya, 46, said he had just dropped off 21 students and eight teachers from Nan for a field trip in Chiang Mai. However, after the bus's engine stopped in front of the nursing home, he let all passengers out to wait for another bus.

As the driver did not secure the wheels, the bus slid down from the mountain, rendering the brakes inoperable.

Despite attempting to restart the engine to brake the bus, the driver was caught up in the accident, resulting in the bus rolling for half a mile.

After the incident, Mae Taeng Police Station apprehended Surachat for further legal action.