A Chinese stock strategy that worked in 2019 when tensions got heated with the US is picking up again, showing mounting worry over the viability of a trade deal signed just months ago.

Miners of rare earths snapped back to life in May, with China Minmetals Rare Earth’s 27% rally leading double-digit monthly gains across the sector. Used in products from electric vehicles to military hardware, prices of the metals surged last spring on speculation China would use it as a bargaining chip in its trade war with the U.S. China produces about 70% of the world’s mined rare earths.

Conflict between the countries has increased the past few months on issues from the coronoavirus to Huawei Technologies Co. to Hong Kong. On Friday, US President Donald Trump announced planned action in response to China’s crackdown on Hong Kong. The battles have raised concerns about the trade deal possibly failing.

“There’s short-term logic behind the play,” said Ji Yongfeng, a fund manager at Shanghai Raymond Asset Management Co. “Rare earths are a necessity in some of the most important sectors, especially new-energy cars, so it’s not entirely a speculative trade.”

The sector surged in May last year on Trump’s initial tariff threats. A month later, China said it was looking at export controls on rare earths, though no measures have been implemented to date. The pandemic may slow US efforts to cut reliance on China for supplies.

Recently, dysprosium oxide jumped 10% in about a week to the highest since August and terbium oxide climbed 7.7%, according to commodities market information provider Shanghai SteelHome E-Commerce Co. The two are among the 17 rare earth elements that are actually abundant in the earth’s crust, but in concentrations generally less conducive to mining than other ores.

“The gaming between China and the US will boost the value of rare earths as a strategic material,” said Shang Li, an analyst with Citic Securities. “We are bullish on the sector.”