Chinese diplomatic and military staff went on buying sprees for illegal ivory while on official visits to East Africa, sending prices soaring, an environmental activist group said Thursday.

Tens of thousands of elephants are estimated to be slaughtered in Africa each year to feed rising Asian demand for ivory products, mostly from China, the continent's biggest trading partner.

When Chinese President Xi Jinping visited Tanzania in March 2013, members of his government and business delegation bought so much ivory that local prices doubled to $700 per kilogram, the UK-based Environmental Investigation Agency (EIA) said in a report, citing ivory traders in the city of Dar es Salaam.

"When the guest come, the whole delegation, that's then time when the business goes up," the EIA quoted a vendor named Suleiman as saying.

The traders alleged that the buyers took advantage of a lack of security checks for diplomatic visitors to smuggle their purchases back to China on Xi's plane.

Similar sales were made on a previous trip by China's former President Hu Jintao, the report said, adding that Chinese embassy staff have been "major buyers", since at least 2006.

A Chinese navy visit to Tanzania last year by vessels returning from anti-piracy patrols in the Gulf of Aden "prompted a surge in business for Dar es Salaam-based ivory traders", it said.

A Chinese national named Yu Bo was arrested during the naval visit as he attempted to enter the city's port in a lorry containing 81 elephant tusks -- hidden under wooden carvings -- which he planned to deliver to two mid-ranking Chinese naval officers, the EIA said.

Yu was convicted by a local court in March and sentenced to 20 years in jail, it added.

- Key China ally -

Tanzania, which has large reserves of natural gas, is a key ally of China in East Africa, and its President Jakaya Kikwete reportedly signed deals with the Asian giant worth $1.7 billion while on a visit to Beijing last month.

Tanzania had about 142,000 elephants when Kikwete took office in 2005, the EIA said, adding that by 2015 the population is likely to have plummeted to about 55,000 as a result of poaching.

Almost all ivory sales were banned in 1989 by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), to which both China and Tanzania are signatories.

Politicians from Tanzania's ruling Chama Cha Mapinduzi (CCM) party and well-connected business people are also involved in the ivory trade, with most demand coming from China, the EIA said.

The EIA report did say that enforcement of the ban on ivory sales had slightly improved last year, with smuggling syndicates growing "more cautious", after Yu's conviction, as well as a high-profile raid.

Police found 706 ivory tusks weighing over 1.8 tonnes at a house in Dar es Salaam last November, along with three Chinese nationals who were detained at the scene after trying to pay a $50,000 bribe, the EIA said.

Meng Xianlin, a Chinese forestry administration official who oversees Beijing's commitments under CITES told AFP that the claims made in the EIA's report were "baloney".

"I have not heard of such a matter," he said, adding: "Do not hype this up."

China often says that it pays "great attention", to the protection of endangered wildlife, and in recent years has carried out several high-profile arrests of smugglers caught in its territory, along with a televised incineration of seized ivory.

Foreign Ministry spokesman Hong Lei described the report as "groundless" at a regular briefing in Beijing Thursday, adding that China was "strongly dissatisfied" with it.

"We attach importance to the protection of wild animals like elephants," he said.

"Recently, in light of the illegal actions of poaching and smuggling of elephant tusks, the Chinese government enacted a series of laws and regulations."

The environmental group WWF estimated that around 25,000 African elephants were hunted for ivory in 2011, predicting that the toll would rise. There could be as few as 470,000 left, according to the group.