The number of African elephants slaughtered for their ivory fell in 2013, according to new official data, but the level of killing is still reducing populations and threatening the survival of the species.

The new figures, from the Convention on International Trade in Endangered Species (Cites), reveal that at least 20,000 elephants were poached in 2013, compared to 22,000 in 2012 and 25,000 in 2011. The report also found rising demand in China for mammoth ivory, which is removed from fossil carcasses in Siberia.

"Although the sharp upward trend in illegal elephant killing observed since the mid-2000s, which peaked in 2011, is levelling off, poaching levels remain alarmingly high," said the report. "It continues to far exceed the natural elephant population growth rates, resulting in a further decline in elephant populations across Africa."

There are about 500,000 African elephants remaining, a fall of 95% over the last century.

John Scanlon, secretary-general of Cites, said: "Due to the collective efforts of so many, we see some encouraging signals, but experience shows that poaching trends can shift dramatically and quickly, especially when transnational organised crime is involved.

"The momentum generated over the past three years must now translate into deeper and stronger efforts to fight these crimes on the frontline, where it is needed most – from the field, to customs, to illicit markets, and only then can we hope to reverse the devastating poaching trends of the past decade."

The slaughter of elephants, rhino, tigers and other species has surged in the past decade, driven by a lucrative illicit trade estimated by Interpol to be worth $10-20bn (£6-12bn) a year. Only drugs, people and arms trafficking earn more for criminals, and the corruption and violence accompanying wildlife crime takes a heavy toll on local communities.

Large seizures of ivory (more than 500 kg) indicate the involvement of transnational organised crime, according to Cites. Major seizures in Africa exceeded those made in Asia, where it was sold, for the first time in 2013. The shift shows enforcement officers are getting closer to the source of the ivory. But, as a result, criminal gangs are now shifting smuggling routes to go via Nigeria, Angola, Mozambique, Spain, Egypt, and Gulf states in the Middle East and are using existing drug trafficking routes.

A herd of elephants help a calf up a slope after crossing the Ewaso Nyiro river in Samburu game reserve in Kenya. Photograph: Carl De Souza/AFP/Getty Images

Three African countries — Kenya, Tanzania and Uganda — accounted for 80% of the major seizures in Africa. Along with China, Malaysia, the Philippines, Thailand, and Vietnam, they make the so-called "gang of eight" identified as most implicated in the illegal ivory trade as source, transit or destination countries. The eight have all been challenged by Cites to implement action plans to tackle the elephant poaching and smuggling crisis.

Robbie Marsland, UK director of International Fund for Animal Welfare, said: "Killing elephants to fuel the ivory trade has grown out of control in recent years. In order to save elephants and other vulnerable species, we need to address every link in the illegal wildlife trade chain which means stopping the killing, stopping the trafficking and stopping the demand."

The Cites report also found a growing trend, primarily in mainland China and Hong Kong, for high-end investment in mammoth ivory from Siberia, which can be legally sold. Almost 100 tonnes was exported in 2013, almost all from Russia.

Ivory trading was made illegal in 1989 under the Cites, but poaching has continued. There have been a series of high-profile destructions of seized ivory in recent months, aimed at curbing demand for the product.

In November, the US crushed six tonnes of illegal ivory. Then, in January, the Chinese government conducted its first destruction of ivory, crushing the same amount. France crushed three tonnes in a ceremony at the foot of the Eiffel Tower in February. In May, Hong Kong began destroying nearly 30 tonnes of ivory seized from smugglers.

Earlier in June, Prince William and David Beckham launched a campaign aimed at stigmatising ivory, while the Wildleaks whistleblowing site revealed it had begun three major ivory investigations based on tip-offs received in its first three months.