Eight Cambodian farmers were killed and one was critically injured on Friday after their truck hit two old anti-tank mines in northwestern Cambodia, police said.

"Seven people died on the spot. One man passed away later in hospital. The driver is in critical condition," Malai district police chief Sao Bun said after the blast on a cassava farm in Banteay Meanchey province.

"Because of heavy rainfall recently, the land became soft and when they drove across the farm they hit two anti-tank mines stacked on top of each other," he told AFP, adding that two other landmines had been found on the grounds in the past four years.

The police chief was unable to give the ages of the victims but they are thought to include at least three teenagers.

Nearly three decades of civil war have left impoverished Cambodia one of the world's most heavily mined countries.

In 2011, leftover landmines and other unexploded ordnance killed 43 people and caused 168 injuries, including 33 amputations, according to official statistics.