Image copyright AFP Image caption Campaigners say rising demand in Asia is fuelling the poaching of elephants in Africa and the smuggling of ivory

Officials travelling to Tanzania with Chinese President Xi Jinping went on a buying spree for illegal ivory, an environmental activist group has said.

The delegation bought so much ivory prices in the local market soared, the Environmental Investigation Agency (EIA) said in a report.

Tanzania rejected the report as "stupid nonsense". China said the allegations were "groundless".

Conservationists say demand for ivory in China is fuelling poaching.

In recent years, poaching has increased across sub-Saharan Africa, with criminal gangs slaughtering elephants for ivory.

Tanzania is the largest source of poached ivory in the world, according to the EIA.

'Security checks averted'

The EIA report cited a trader in Tanzania's main port city, Dar es Salaam, named as Suleiman Mochiwa, who met undercover investigators.

He said that when the Chinese government and business delegation arrived, ivory prices in the local market doubled to $700 (£438) per kilo during the visit.

Image copyright AP Image caption Earlier this year China destroyed a large quantity of confiscated ivory for the first time

"The [delegation]... used the opportunity to procure such a large amount of ivory that local prices increased," the report says.

Investigators alleged that the Chinese buyers could take advantage of a lack of security checks for those in the country on a diplomatic visit.

"The two traders claimed that a fortnight before the state visit, Chinese buyers began purchasing thousands of kilos of ivory, later sent to China in diplomatic bags on the presidential plane," the report added.

"When your president [Xi Jinping] was here… many kilos go out… many kilos. Half of his plane go with that," one of the traders told the EIA investigators.

The trip was Xi Jinping's first foreign tour as head of state. Traders told the group that similar ivory sales took place on an earlier trip by China's former President Hu Jintao.

Analysis: Celia Hatton, BBC News, Beijing

The illegal ivory trade is flourishing in China, where many prize ivory carvings as valuable status symbols.

However, a portion of Chinese society, including some parts of the government, is working to eradicate illicit ivory sales.

The country's state media publicises the arrests of smugglers and, earlier this year, the first televised destruction of confiscated ivory.

However, education campaigns have a long way to go.

Some in China don't realise that one has to kill an elephant in order to harvest its tusks. In Chinese, the term for ivory is translated literally as "elephant tooth", leading a sizeable portion of the population to believe, in error, that elephants can re-grow their tusks.

Complicating the issue is that China allows limited sales of legal ivory. Conservationists, both inside China and outside its borders, argue that the government needs to ban sales completely in order to stop the trade in its tracks.

'Not believable'

"The report is groundless, and we express our strong dissatisfaction," Chinese foreign ministry spokesman Hong Lei is quoted as saying by the Associated Press news agency.

The director of China's endangered species import and export management office also dismissed the claims: "Allegations without evidence are not believable," Meng Xianlin said.

A spokesmen for the Tanzanian government, Assah Mwambene, said the EIA was trying to "frustrate" efforts to halt the ivory trade. "Everybody knows we have been at the forefront of fighting this illegal trade."

The ivory trade was banned in 1989 by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (Cites). Both China and Tanzania are signatories.

China does have about 150 legal, government-licensed ivory shops, which sell ivory collected prior to this. They are the only places allowed to sell ivory to individual buyers.

Earlier this year China for the first time destroyed a large quantity of confiscated ivory, in a public event described by conservation groups as a landmark move.

Just over six tonnes of carvings, ornaments and tusks amassed over the years were fed into crushing machines.