The latest surge in prices came two days after Chinese President Xi Jinping visited a rare earth company in Jiangxi, sparking speculation that China, the world's dominant producer, could restrict exports to the United States as part of a trade war between the two countries.
Customs in southwest China's Yunnan province, which borders Myanmar, has prohibited rare earth mineral imports from the Southeast Asian country from May 15, the state-run Securities Times reported on May 13.
Myanmar accounted for 50% of China's medium-heavy rare earth feedstock in 2018, the newspaper said. Analysts believe the ban is an attempt to stop smuggling.
Chinese prices for the metal form of dysprosium, one of the middle-heavy rare earths used in permanent magnets, rose by 16.4 percent to as much as 2,300 yuan ($333.25) per kg, the highest since May 2015, on Wednesday and are up more than 25 percent since the Securities Times report was published.
Terbium metal has soared to 4,500 yuan per kg, the highest since November 2017, and gadolinium oxide prices are up 19 percent this month, hitting 175,000 yuan per tonne on Wednesday, the highest since August 2017.
"Prices of middle and heavy rare earths have enjoyed steady gains since the start of the year," Ryan Castilloux, managing director of Adamas Intelligence, said in an email. "In recent days the rally has turned to a race on the back of the import ban from Myanmar and ongoing trade war-related speculation."
Shares in companies that make rare earth products, used in everything from mobile phones and medical equipment to jet engines and electric vehicles, have soared since Xi's visit. Australia-listed Lynas Corp, the only major rare earths producer outside of China, which stands to gain from the Myanmar ban, hit a six-month peak on Wednesday.
Yunnan customs referred questions on the Myanmar ban to the General Administration of Customs in Beijing, which did not respond to a faxed request for comment.
(Reporting by Tom Daly; editing by David Evans)