Poaching is inflicting unsustainable losses on African elephants, with more than 33,600 animals killed each year between 2010 and 2012, a study has shown.

Over that period alone an estimated 6.8% of the continent's elephant population was wiped out by the illegal ivory trade, scientists say.

In central Africa, the worst affected region, poaching led to a 63.7% fall in elephant numbers between 2002 and 2012.

Evidence suggests a strong link between the slaughter, the price of ivory and increasing demand in China where carved ornaments made from elephant tusks are highly prized.

Researchers writing in the journal Proceedings of the National Academy of Sciences suggested that the very existence of the African elephant was threatened.

They wrote: "Our analysis demonstrates the heavy toll illegal ivory trade is taking on African elephants, and suggests current off-take exceeds the intrinsic growth capacity of the species."

The team led by Dr George Wittemyer, from Colorado State University, began by conducting a survey of elephant carcasses in Kenya's Samburu National Reserve.

Poaching rates in Samburu were strongly associated with local market price increases for ivory and larger number of seizures of ivory bound for China.

Carcass survey data covering 45 sites throughout Africa were then used to extrapolate these findings on a continental scale using two different statistical methods.

The results showed that illegal killing was most pervasive in central Africa.

While savannah elephant populations in east and southern Africa were relatively stable or growing between 2002 and 2009, they then went into decline.

Overall, the slaughter peaked in 2011 with 8% of Africa's elephants illegally killed - a total of more than 40,000 animals - leading to a probable species reduction of 3% that year alone.

The researchers concluded: "These results provide the most comprehensive assessment of illegal ivory harvest to date and confirm that current ivory consumption is not sustainable."