Ukraine open to Trump’s idea to exchange aid for rare earths—But there’s a catch

As a result, access to valuable Ukrainian natural resources will depend, at least in part, on the battle for eastern Ukraine, where Russian forces are currently advancing slowly but steadily.

That’s especially true for rare earths, a collection of minerals that have become essential in certain high-tech industries, including defense and renewable energy—in which Trump has expressed particular interest. Currently, the majority of the world’s supply of rare earths come from China.

“We’re looking to do a deal with Ukraine where they’re going to secure what we’re giving them with their rare earths and other things," Trump said, speaking at the White House on Monday.

Though rare earth deposits have been found in several parts of Ukraine, by far the largest known field is in territory that spans the frontline in eastern Ukraine, according to maps published by the Ukrainian Geological Survey, the state regulator for mineral resources.

Ukraine also has substantial deposits of other valuable minerals, including lithium, cobalt and titanium. Ukraine says it has Europe’s largest reserves of titanium, which is used to make alloys used in aircraft and naval vessels and lithium, which is used in batteries.

The plan to offer Western allies access to Ukraine’s mineral resources in exchange for continued military and financial support was first outlined by Ukrainian President Volodymyr Zelensky, who made it part of what he called a “Victory Plan" that he presented to Trump and U.S. officials last fall. Speaking on Tuesday, Zelensky reiterated his willingness to let the U.S. develop his country’s resources if they would keep shipments of arms flowing to Kyiv.

“Russia has been occupying our lands since 2014, and some of those areas contained significant reserves of our natural resources," Zelensky told reporters. “We are open to developing these resources with our partners—the ones who help us defend our land and push the enemy back with their weapons, presence, and sanction packages. This is absolutely fair."

Trump’s proposal of exchanging resources for military aid echoes comments in his first term that Afghanistan’s vast mineral wealth could pay for the war there.

Western experts expressed some skepticism that some of Ukraine’s minerals—including rare earths—could be mined anytime in the near future.

“The biggest flaw with this plan is that most reserves are located in areas of Ukraine that are under Russian control or very close to front lines, meaning no one will be able to mine and process the materials," said Wolf-Christian Paes, a senior fellow for armed conflict at the International Institute for Strategic Studies. “These assets will be hard to access without a lasting peace in Ukraine. A ceasefire isn’t enough."

In addition, rare earths are very expensive to mine, and the known deposits in Ukraine are much smaller than those in the U.S., said George Ingvall, a price analyst at Benchmark Minerals Intelligence.

Vadym Denysenko, a political analyst and former member of Ukraine’s parliament, said that the idea would be to open Ukrainian resources to Western allies after a peace deal had been reached.

“Yes, we have some big sources of minerals in occupied territories, but we also have a lot of them not in occupied territories," he said. “If you have a real peace deal—after that it will not be a problem."

Aid from the U.S.—including long-range missiles, advanced air-defense systems and a steady supply of artillery shells—has been an essential factor helping Ukraine defend itself against the Russian onslaught for nearly three years. If Trump were to cut off arms deliveries—as he suggested he might during the campaign—Ukraine could soon be at risk of a military collapse.

Although Trump is now signaling a willingness to continue support for Kyiv—as long as there is something in it for the U.S.—he has also talked about lessening the financial burden of funding Ukraine’s defense. As well as floating the idea of trading aid for access to rare-earth minerals on Monday, Trump also criticized European allies for not shouldering a larger share of the assistance.

“They’re way below us in terms of money," he said. “They should be paying at least equal."

Kremlin spokesman Dmitry Peskov characterized Trump’s comments about a deal for Ukrainian resources as an offer for Ukraine to buy assistance, instead of receiving it for free.

“Of course, what would be better is not to provide assistance at all," Peskov told reporters on Tuesday, according to the Russian state news agency TASS.

Louisa Naks, Nikita Nikolaienko and Kate Vtorygina contributed to this article.

Write to Ian Lovett at ian.lovett@wsj.com