SHANGHAI — President Xi Jinping of China strode this week through a high-ceilinged factory that makes magnets out of rare earths, minerals that are essential to global manufacturing and a sector that his country dominates. His top trade negotiator, Vice Premier Liu He, stood near.

Xi did not threaten to block supplies of rare earths to the United States. He didn’t have to. The veiled threat, broadcast over state-run news media this week as President Donald Trump ratcheted up his trade war against Beijing, was clear.

China’s command of the rare-earth market could give Beijing a way to strike back at Trump as he raises tariffs and deprives Chinese companies of the technology they need to survive. A similar move by China nine years ago, against Japan over a territorial dispute, shocked manufacturers around the world, sent prices soaring and revealed Beijing’s control of an essential part of the global supply chain.

This time, however, the impact of any block on rare earths may be far less clear-cut. It could undermine China’s reputation as a manufacturing hub. Other trading partners, notably Japan and South Korea, could become collateral damage. And in an odd turnabout, China’s own needs have made it somewhat dependent on ore from the United States.

While China is determined to resist American pressure, limits on rare earths “will affect many other countries,” said Gary Liu, a Shanghai-based economist. “The global supply chain is so complicated.”

It is far from clear that China will harness rare earths as a weapon. On Wednesday, a spokesman for the Chinese Foreign Ministry advised reporters not to read too much into Xi’s visit Monday to the magnet factory in Jiangxi province. Hours later, Hu Xijin, the editor of Global Times, a tabloid owned by the Chinese Communist Party, said leaders in Beijing were considering the idea.

“I think Chinese government won’t do this immediately,” Hu wrote on Twitter, “but it’s seriously evaluating the need to do so.”

Rare earths are not actually rare. But refining them from ore is expensive and polluting.

China has been one of the very few countries willing to tolerate the industry. Though rare-earth mines have opened in the United States, Australia and elsewhere, China dominates refining and transforming them into valuable metals, magnetic powders and other high-value products.

The minerals wind up in everything from iPhones to wind turbines and missiles. They are used to polish camera lenses and to refine crude oil into gasoline, diesel and jet fuel.

The appetite in the United States for products that include rare earths is enormous. But while the country still imports large quantities of cheap rare-earth catalysts for use in oil refineries, American demand for raw rare-earth metals to be used in factories has almost disappeared. Chinese customs data show that the United States bought only 3.8 percent of China’s exports of rare-earth metals last year, far less than Japan, and also less than India, Italy or Spain.

In large part, that is because so much manufacturing has shifted out of the United States. Nearly a decade ago, Beijing began putting heavy pressure on manufacturers of products like electric motor magnets and light-emitting diodes to move factories to China if they wanted reliable access to rare-earth metal supplies. Remaining US industries like automaking and aerospace manufacturing now import entire systems from China, like car starters and aircraft wing flaps.

Beijing could still block exports of Chinese-made motors, magnets and other gear to the United States — and industry experts noted that Xi’s visit was to a magnet factory, not to a mine.

“The message was, this is a supply chain and we control your supply chain,” said Clint Cox, president of the Anchor House, a rare-earths consulting firm in Evanston, Illinois.

The dilemma for Beijing lies in whether to jeopardize its central role in global supply chains by halting exports of crucial components to the West. Trade hawks in the Trump administration have been quietly expressing hope that China will do just that. They see such an interruption as the best way to persuade global companies to shift manufacturing permanently out of China to the United States or to US allies, a long-term goal known as decoupling in trade circles.

A Chinese export embargo would have other drawbacks. For example, US oil refineries depend on lanthanum, which is the cheapest and most easily produced of the 17 rare-earth elements, as a catalyst to refine crude oil. But lanthanum is mined in bulk in Australia and in the United States as well as in China.

Oil companies in the United States keep several months of catalysts in stockpiles, said Dudley Kingsnorth, a professor specializing in rare earths at the Western Australian School of Mines in Perth. The United States could import more gasoline and diesel from refineries elsewhere if needed, although at a greater cost.

Thanks to circuitous global supply chains, really blocking US access to rare-earth products could mean cutting off much of the rest of the world as well. Factories in South Korea and Thailand produce large quantities of lanthanum-based catalysts. Three Japanese companies dominate the business of turning rare earths into magnets. The three — Hitachi, TDK Corp. and the Shin-Etsu Group — have built large magnet factories in China, but have kept their factories open in Japan as a precaution.

In an odd and little-noticed reversal, China has actually become somewhat dependent on the United States for rare-earth ore. China’s manufacturing sector is now so huge that the country has begun importing semi-processed rare-earth ore from a mine in Mountain Pass, in the California desert near the Nevada border.

The mine went bankrupt in 2015 because it could not compete with illegal mines in China that have few environmental controls and low costs.

But its new owners have shipped part of its stockpiles and all of its current ore production to China for processing. In recent months, the mine has accounted for roughly one-tenth of the world’s rare-earth mining.

A group of investors led by JHL Capital Group, a Chicago hedge fund, bought the mine out of bankruptcy in July 2017. JHL Capital owns almost 65 percent of the mine, and another U.S. investment group, QVT Financial of New York, owns a little over 25 percent.

Shenghe Resources Holding Co. of China owns the remaining shares, but those shares do not carry voting rights, said James H. Litinsky, the founder and controlling shareholder of JHL. Shenghe provides technical and sales advice, but the mine’s 200 employees are Americans, he said.

Shenghe’s majority shareholder is an institute controlled by China’s Ministry of Land and Resources. Shenghe has publicly confirmed its stake in the mine.

Mining the rare earths is only part of the equation. Litinsky said that he planned to restart the Mountain Pass mine’s mothballed chemical separation facilities next year to produce rare-earth oxides, so that semiprocessed ore would no longer have to be shipped to China. That plan is based partly on his assessment that trade frictions will persist and that the United States will seek self-sufficiency.

“This is the very beginning of a multidecade transformation of the global economy,” he said. “The global economy is going to bifurcate into the US bloc and the China bloc.”

When it comes to rare earths, an American bloc would be hard pressed to catch up, despite the reopening of Mountain Pass. China so completely dominates one key stage of the manufacturing process — converting the oxides to metals — and has so much low-cost overcapacity that companies elsewhere are leery of investing in their own facilities.

“We’re a long way off,” said Cox of Anchor House. “We’re nowhere.”

2019 New York Times News Service