XI'AN -- Three policemen were killed in a traffic accident in northwest China's Shaanxi province Wednesday, the local authority said.

The accident took place in Wuqi county of Yan'an city at about 3 pm when a tanker carrying cement rammed a police wagon patrolling the area, said Yan Wenxue, a publicity officer with the county government.

Three of the four police officers in the wagon were killed. One died on the spot and two died later in hospital.

The injured policeman was in stable condition, doctors said.

Police were questioning the driver of the tanker for the cause of the accident.

Wuqi is about 7 hours' drive from Xi'an, capital of Shaanxi.