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 USD 700 per kilogram, the UK-based Environmental Investigation Agency (EIA) said in a report, citing ivory traders in the city of Dar es Salaam.
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.
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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.
Yu was convicted by a local court in March and sentenced to 20 years in jail, it added.
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 USD 1.7 billion while on a visit to Beijing last month.
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.