China's booming e-commerce websites have carried thousands of advertisements for illegal wildlife products including ivory, rhino horn and tiger bone, a wildlife trade monitoring network said Tuesday.

More than half of such products offered online in recent months are ivory, the British group TRAFFIC found in a survey of 15 Chinese retail websites over a two year period.

Conservationists say China is the world's largest consumer of illegal ivory, with skyrocketing demand fuelling the slaughter of tens of thousands of African elephants each year.

TRAFFIC found that surveyed websites were hosting thousands of advertisements for illegal wildlife products as of the end of 2014.

It had consistently found "around 1,500" new advertisements for such items every month for the last two years, it said in a report.

As well as ivory, it tracked offers of rhino horn, leopard and tiger bones, hawksbill turtle shells, pangolin scales, hornbill casques and the horns of the saiga, a critically endangered antelope.

China in 2013 overtook the United States to be the world's largest online retail market, with firms such as Alibaba among the most prominent.

To avoid clampdowns online vendors used codewords such as "African materials", "yellow materials" and "white plastic" in place of ivory, said TRAFFIC, which did not specify the sites it monitored.

It follows a total of 64 codewords for the various items being offered.

"There are transactions of illegal wildlife products which were not captured by TRAFFIC's monitoring," it said. "This indicates that trade-volumes on e-commerce platforms might be even higher."

But at "fewer than 10,000" the running total was a "dramatic drop" from more than 50,000 advertisements found when it began monthly surveys in 2012, it added.

Several of the country's leading e-commerce platforms stated they had a "zero-tolerance" policy towards illegal wildlife trading, and removed some listings following complaints, TRAFFIC added.

A joint report in December from Save the Elephants and The Aspinall Foundation campaign groups found that more than 100,000 wild elephants were killed from 2010 to 2012, with the slaughter largely fuelled by the "out of control" illegal ivory trade in China.

Beijing has made efforts to curb the trade, stepping-up prosecutions of smugglers and seizures of ivory at border posts, but campaigners say the measures have not gone far enough.

Last week China announced a one-year ban on imports of ivory carvings, but activists described the move as symbolic as legal imports are minor and most seizures of illegal items are of raw ivory.