Chinese officials and businesspeople used a state trip by President Xi Jinping and other high-level visits to smuggle ivory out of Tanzania, an environmental watchdog said Thursday, casting doubt over Beijing's efforts to end 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 (EIA) 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 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 Ministry of Foreign Affairs denied the report. Spokesman Hong Lei said at a daily briefing that China has "consistently" opposed poaching and has sought to crack down on ivory smuggling.

"The report is groundless, and we express our strong dissatisfaction," Hong said.

Meng Xianlin, director general of the Endangered Species Import and Export Management Office of China, said he has never heard of the 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 after legal pieces started showing up in shops, ivory increasingly became a status symbol in China. Carved ivory has historically been highly prized in the country, and its scarcity has turned it into an investment choice akin to gold and silver.