President Trump has described a potential multibillion-dollar weapons sale to Taiwan as a “negotiating chip” with China, raising new doubts about the pace and scale of American military support for the island democracy.

Taiwan’s government has been waiting for months for Mr. Trump to sign off on a $14 billion package of missiles, anti-drone equipment and air-defense systems intended to fortify the island against Beijing’s military threats.

Mr. Trump himself had pressured Taiwan to spend more on its own defense. Now he is using the very arms his administration had pushed the island to buy as leverage with China, the United States’ main adversary.

Mr. Trump told reporters on Air Force One after leaving China on Friday that he had discussed the weapons package with China’s president, Xi Jinping, during their summit this past week in Beijing. He was asked in an interview with Fox News whether he would approve the Taiwan deal.

“No, I’m holding that in abeyance and it depends on China,” he said in the interview, which was recorded in Beijing but aired after he left. “It depends.”

“It’s a very good negotiating chip for us, frankly,” he said. “It’s a lot of weapons.”

He did not go into details about what he wanted in return, but Mr. Trump has pushed China to make major purchases of American airplanes, ethanol, soybeans, beef and sorghum.

His comments appear to undermine the assurances to Taiwan from some in his own administration that U.S. support for the island is steadfast and nonnegotiable. Before the summit, a bipartisan group of senators had urged against letting support for Taiwan become a bargaining chip with China.

“It looks increasingly likely that Trump will indefinitely withhold the $14 billion arms package to Taiwan, in the hopes that Beijing will give him what he wants on the economic front,” said Amanda Hsiao, a China director at Eurasia Group, a consulting firm.

Raising the Pressure on China

By saying his approval of the arms deal “depends on China,” Mr. Trump seemed to be suggesting that the ball now lay in its court. There was no immediate comment from Beijing on Mr. Trump’s remarks.

On the first day of their talks in Beijing, Mr. Xi told Mr. Trump that the “Taiwan issue is the most critical issue in China-U.S. relations.” If the matter was mishandled, Mr. Xi warned, that could put “the entire U.S.-China relationship in an extremely dangerous situation.”

Mr. Trump also seemed keen to show that he listened intently to Mr. Xi’s views, saying in the interview that after a long talk with the Chinese leader, “I think I know more about Taiwan right now than I know about almost any country.”

But he also later raised the possibility that he might call Taiwan’s president, a step sure to enrage Beijing.

“I have to speak to the person that right now is — you know who he is — that’s running Taiwan,” he said, in a possible reference to the island’s president, Lai Ching-te.

Should Mr. Trump do so, he would be the first sitting American president known to have spoken to a Taiwanese leader since at least 1979, when the United States severed diplomatic ties with Taiwan as part of its recognition of the People’s Republic of China. China objects vehemently to any contact between senior U.S. and Taiwanese officials.

Mr. Trump’s gambit could backfire, either by angering Beijing if he approves the arms sales, or by possibly prompting American lawmakers to step up pressure on him to increase support for Taiwan.

If Mr. Xi wants to punish the Trump administration over Taiwan, analysts have said, China could hold back on orders of farm goods, or ramp up restrictions on exports of rare earths that are essential to many technology components. But Mr. Xi also agreed to make a state visit to the United States this year, and could use the prospect of more talks — and more deals — to influence Mr. Trump.

A Blow to the Taiwan President’s Agenda

Mr. Trump’s comments are likely to be seen as a setback for Mr. Lai of Taiwan, who had pushed strenuously for more military spending and purchases of American weapons.

When Taiwanese lawmakers finally voted for $25 billion in special funding to pay for the two weapons packages from the United States, lawmakers from Mr. Lai’s own party abstained from the vote because it left out spending for domestically made drones and other weapons.

Taiwan’s government tried to quickly smooth over any tensions, saying that they have been assured multiple times by American officials that U.S. policy remains unchanged.

“Our country is grateful for President Trump’s continued support for security in the Taiwan Strait since his first term,” Mr. Lai’s office said in a statement about Mr. Trump’s latest remarks. Countries near China “were cooperating with the United States to actively strengthen their defenses; Taiwan cannot and will not be an exception.”

Mr. Trump’s remarks would also “provide cheap ammunition” for Mr. Lai’s detractors in Taiwan, who say he is too beholden to Washington, said William Yang, a senior analyst in Taiwan for the International Crisis Group, which monitors and tries to help resolve conflicts.

Some politicians from Taiwan’s main opposition party, the Nationalist Party, which favors stronger ties with China, quickly came out to say that Mr. Trump’s comments showed Mr. Lai had been gullible.

Under Mr. Lai, Taiwan has sought to build a more nimble, mobile military with missiles and air-defense technology that can counter China’s expanding military, in large part by buying U.S. weapons. Mr. Trump already approved one $11 billion package last year, a move that Beijing responded to with military exercises near the island.

As for the $14 billion package, Mr. Trump would only say that he would “make a determination over the next fairly short period of time.”

In his interview with Fox, Mr. Trump also revived his longstanding accusation that Taiwan had gained its world-leading know-how in semiconductor production in underhanded ways, and therefore owes the United States. He also emphasized its vulnerability to attack from China, with the United States many thousands of miles away.

“They stole our chip industry,” Mr. Trump said of Taiwan. “Taiwan would be very smart to cool it a little bit. China would be very smart to cool it a little bit.”

Does Trump Believe Beijing?

Perhaps more worrisome for Taiwan’s government is that Mr. Trump’s account of his conversations with Mr. Xi suggested that he took onboard China’s argument that Taiwan bore a big part of the blame for the tensions. China has cast Mr. Lai and his officials as dangerous separatists who are trying to drag the United States into a bruising war.

“Well, it’s a risky thing when you go independent, you know,” he said. “They’re going independent because they want to get into a war, and they want to, they figure they have the United States behind them.”

“I’d like to see it stay the way it is,” he said, apparently referring to Taiwan’s so-called status quo, in which the island is functionally separate but not pursuing formal independence.

Taiwan, which has asserted that China is the aggressor, has never been ruled by the Chinese Communist Party. Most Taiwanese see themselves and their island democracy as distinct from China, and they have no desire to be brought under Beijing. Mr. Lai and his party reject Beijing’s claims over Taiwan and say the island is already in effect independent.

Mr. Trump’s comments “suggest that Xi’s presentation on Taiwan, which all but certainly framed Taiwan as the source of cross-strait tensions and a nonexistent Taiwanese push for independence as the key risk that needs to be managed, had an effect on Trump,” said David Sacks, a fellow at the Council on Foreign Relations who studies China.