Chinese officials used a state trip by President Xi Jinping and other high-level visits to smuggle ivory out of Tanzania, an environmental watchdog said in a report Thursday that cast doubt over Beijing's efforts to stamp out the illegal trade that has led to rampant elephant poaching throughout Africa.

China is the world's largest importer of smuggled tusks, and Tanzania is the largest source of poached ivory, the London-based Environmental Investigation Agency said. Poaching in Tanzania alone has killed half of the country's elephants in the past five years, the group said in the report.

It said that Chinese-led criminal gangs conspired with corrupt Tanzanian officials to traffic huge amounts of ivory, some of which was loaded in diplomatic bags on Xi's plane during a presidential visit in March 2013.

China's foreign affairs officials did not immediately respond to a request for comment on the report, which came at a time when the Chinese government has openly denounced illegal ivory trade.

But Meng Xianlin, director general of the Endangered Species Import and Export Management Office of China, said Thursday that he has never heard of involvement of Chinese delegations in ivory trade.

"I don't think there's hard evidence, and I have not seen such cases," Meng said. "Allegations without evidence are not believable, and I don't think it is appropriate for (EIA) to come up with this mess."

He said that the EIA has been "unfriendly to China for quite some time," calling the allegations irresponsible.

The illicit trade began to explode in China in 2008, when Beijing was permitted to purchase 62 tons of ivory under the Convention on International Trade of Endangered Species. The purchase was presented as a way to keep alive China's traditional artisan ivory carving industry. A state-owned enterprise was authorized to sell the legal ivory to about 200 licensed factories and vendors.

But critics say ivory soon became a status symbol of choice in China after legal pieces started showing up in shops. The legal purchases of ivory provided a convenient cover for a thriving black market in recent years.

The country's licensing system is flawed and enforcement is lax, said Grace Ge Gabriel, Asian regional director for International Fund for Animal Welfare. On top of that, the ivory-buying public in China is largely unaware that the global ivory trade is banned and that elephants must be killed in order to obtain tusks. Many are simply indifferent to the animal plight on a distant continent, she said.

In its report, EIA said its investigators learned as early as 2006 that some staff of the Chinese Embassy in Tanzania were major buyers of illegal ivory.

It said Chinese government officials and businesspeople in an entourage with Xi during the 2013 state visit used the opportunity to buy such a large amount of ivory that local prices doubled. Two weeks before the visit, two traders claimed that Chinese buyers began purchasing thousands of kilograms of ivory, which was later sent to China in diplomatic bags on the presidential plane, the EIA report said.

It said that in December 2013, one dealer boasted of having sold $50,000 worth of ivory to Chinese navy personnel on an official visit in Tanzania's port city of Dar es Salaam. It said a Chinese national was caught trying to enter the port with 81 illegal tusks intended for two Chinese naval officers.

In China, authorities have campaigned against illegal ivory. Six tons of illegal ivory was pulverized earlier this year in the southern city of Dongguan, and Chinese courts have stepped up prosecutions of illegal ivory trade.

Criminal cases involving endangered animals and the illegal ivory trade rose 9.6 percent in the first months of last year, compared with the same period a year earlier. The government has warned Chinese tourists in Tanzania not to purchase ivory products or face stiff penalties.

But animal rights and environmental protection advocates have called on China to ban the ivory trade altogether.

"We are already seeing the detrimental effect to allow a little bit of the ivory trade. We know that does not work," Ge Gabriel of IFAW said. "We certainly hope any country that has a domestic ivory market should shut it down."