President Xi Jinping’s visit to a rare earths facility fueled speculation that the strategic materials could be weaponized in China’s tit-for-tat with the U.S. on trade.

Shares in JL MAG Rare-Earth Co. surged by their daily limit Monday after state news agency Xinhua said the Chinese president had stopped by the company in Jiangxi. Official news outlets give regular updates on the whereabouts of top leaders, sometimes leading to share spikes on the belief that companies have been handed official backing.

But the visits may also flag policy priorities, and rare earths have featured in the escalating trade spat between the U.S. and China. The Asian country raised tariffs to 25% from 10% on American imports, while the U.S. excluded rare earths from its own list of prospective tariffs on roughly $300 billion worth of Chinese goods to be targeted in the next wave of measures.

The U.S. relies on China, the dominant global supplier, for about 80% of its rare earths imports. Xi was accompanied on the trip to JL MAG by Liu He, the vice premier who has led the Chinese side in the trade negotiations.

The visit “sends a warning signal to the U.S. that China may use rare earths as a retaliation measure as the trade war heats up,” Yang Kunhe, analyst at Pacific Securities Co., said by phone from Beijing. That could include curbs on rare earth exports to the U.S., he said.

A stand-off between the U.S. and China deepened in the past few days after the Trump administration on Friday blacklisted Huawei Technologies Co. -- which it accuses of aiding Beijing in espionage -- and threatened to cut it off from the U.S. software and semiconductors it needs to make its products. A spokesman for China’s foreign ministry told reporters Monday to “please wait and see” how the government and companies respond.

Restrictions on rare earth materials from China would hurt its domestic miners. But curbs could potentially help companies like JL MAG, which makes magnets containing rare earths that are used in products including electric vehicles and wind turbines.

“With curbs on exports, the government may need to develop some policies to bolster the domestic industry, as the rare earth industry is also part of the country’s advanced manufacturing plan,” Yang said.