Chinese authorities have earned almost $5 trillion in profit by selling land obtained from farmers to developers over the years, a top economist said according to state media.

As the country undergoes a huge urbanisation process rural land confiscations have led to numerous protests, worrying the ruling Communist Party, which sees corruption and social unrest as threats to its power.

By law, officials may provide compensation worth up to 30 times the value of the land's output, but in practice they have skimped on payments or foregone them altogether -- then sold the land to developers at much higher rates.

Wu Jinglian, an economist at the Development Research Centre of the State Council, China's cabinet, said: "Some (government) agencies have earned around 30 trillion yuan, by conservative estimates, from land price spreads in the urbanisation campaign over the past decades."

The 30 trillion yuan figure equates to $4.8 trillion at current exchange rates.

Wu, a former adviser to Zhu Rongji, premier in 1998-2003, made the remarks at an economic forum at the weekend, the official Xinhua news agency said.