President Trump warned on Thursday that South Korea would pay more, perhaps twice as much, if it did not agree to invest in a natural gas development in Alaska.

Speaking with reporters at the White House, Mr. Trump rejected the suggestion that he had announced the project prematurely, insisting that the Koreans took part in the talks. “They were there, the Korean delegation was there,” he said.

Then came the threat. “We can just charge them more,” Mr. Trump said. “Tell them if they don’t agree, I’ll double it up.”

Mr. Trump had said in September that South Korea agreed to put $50 billion, about 67 trillion won, into the Alaska liquefied natural gas project. The South Korean government immediately pushed back, saying “nothing has been decided about whether to invest or the scale of any investment,” and the two sides have kept talking past each other, as rews reported this week.

The president did not say on Thursday what exactly he would double. But his remark carried the possibility of a larger investment demand or higher tariffs, tools he has reached for before to extract concessions from Seoul, according to the Chosun Ilbo, a South Korean newspaper that reported the exchange.

Doug Burgum, the interior secretary and chairman of the National Energy Council, defended the announcement in a meeting with the Chosun Ilbo on Wednesday in Des Moines, Iowa. Referring to Mr. Trump and the South Korean president, Lee Jae Myung, he asked whether they had not been “in the same room” at their New York meeting and said the arrangement would be “a great deal” for Korea, too.

The Alaska project has now come up twice in a short stretch. On Thursday morning Mr. Trump said on Truth Social that South Korea would invest $8.4 billion in an enhanced oil recovery project there, a commitment that was not part of the two countries’ investment agreement. The announcement left South Korean officials scrambling to check whether it was real, the Chosun Ilbo reported.

In January, Mr. Trump threatened to raise the reciprocal tariff on South Korea to 25 percent from 15 percent, faulting the country’s National Assembly for dragging its feet on a special law governing investment in the United States. The Office of the United States Trade Representative has already imposed a 12.5 percent “forced labor” tariff, citing Section 301 of trade law, and has opened a further investigation into what it calls structural overproduction.

The Alaska squeeze carries a domestic calculation. Republicans need to hold Alaska to keep their Senate majority, and Senator Dan Sullivan, the Republican incumbent, is in a close race against Mary Peltola, a Democrat and former congresswoman. The LNG development has long been a goal of Alaskans, and Mr. Trump has made plain he wants credit for delivering it. At a rally in Oklahoma on Wednesday, he said he planned to travel there: “There’s a great senator there, Dan Sullivan, and we will get him elected.”