More elephants in Africa are being killed by poachers than are born each year, and the problem may be worse than previously understood, according to the most detailed assessment yet, released on Monday.

Using a newly refined approach to estimate elephant deaths, developed at Kenya's Samburu National Reserve, researchers said Africa's elephant population is declining at a rate of about two percent annually.

"Basically, that means we are starting to lose the species," said lead author George Wittemyer, an assistant professor in the department of fish, wildlife and conservation biology at Colorado State University.

While the actual number of African elephants in the wild is difficult to know for certain, the World Wildlife Fund (WWF) estimates there are between 470,000-690,000.

The newly developed model covers the entire continent and therefore shows that the number of elephants that died in recent years is higher than previous estimates.

For instance, experts agree that the most recent peak year for illegal elephant killings was 2011.

According to data from the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), about 25,000 elephants may have been poached across Africa in 2011, based on about four dozen sites being monitored.

This study however, shows that illegal poaching removed about eight percent of the population in 2011, which "extrapolates to around 40,000 elephants illegally killed," when the entire continent is considered, it said.

On average, poaching took an average of 33,630 elephants' lives per year from 2010 to 2012, the study found.

"It has dropped a bit in 2012-2013 but it is still at a rate that is too high and is driving the decline of the species," Wittemyer told AFP.

The new mathematical method was based on more than a decade of studying the natural deaths and illegal killings among elephants in northern Kenya.

The approach was then extended to carcass data from international monitors and extrapolated across the African continent.

"From 2010 to 2012, we calculated that we lost over 100,000 individual elephants. It has just been a total disaster," said Wittemyer.

"Wittemyer and colleagues have taken an important step -- an obvious step in retrospect, but it's difficult to get the right data for such an analysis," said Susan Alberts, an elephant expert and biology professor at Duke University.

"The careful work that Wittemyer and colleagues have done here is badly needed," added Alberts, who was not involved in the study.

A surge in elephant deaths at the hands of poachers also coincided with the spike in the price of black-market ivory bound for sale at Chinese markets, Wittemyer said.

"We found that the rise in poaching was very closely related to the local price of black-market ivory," he told AFP.

"Basically, when the price of ivory got over $30 per kilogram, the killing rate started becoming unsustainable," he added.

"It became really high and a really big problem, and unfortunately that price got up to around $150 per kilogram."

Even more, since poachers tend to aim for the largest elephants with the biggest tusks, the loss of males in their prime breeding years and family matriarchs is upsetting the creatures' social groups and leaving bands of orphans to fend for themselves.

"Thousands of elephant families are being disrupted or destroyed," Wittemyer said.

Elephants are faring better in places like Botswana, Namibia and South Africa, where protections are in place.

However, the outlook is dire for about three-quarters of all elephants, particularly those in central Africa, Tanzania and Mozambique, he told AFP.

The key to reversing the trend is to curb the demand from ivory markets in the Far East, he said.

"Somehow we need to break this cycle, this consumptive cycle, of using ivory as a product in those markets," Wittemyer said.

The study appears in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal.