SEOUL, South Korea — South Korea settled disputes with the Trump administration over steel tariffs and the revision of a free-trade agreement this week. With those stumbling blocks cleared, it hoped the two governments could focus on the more pressing crisis of North Korea’s nuclear weapons.

As it turned out, South Korea was celebrating too early.

On Thursday, President Trump said he might postpone finalizing the trade agreement with South Korea, which he had earlier described as “a wonderful deal with a wonderful ally,” until he secured a deal in denuclearizing North Korea.

“You know why?” he said. “Because it’s a very strong card.”

By tying a trade deal with South Korea to progress in denuclearizing North Korea, Mr. Trump is showing how little direct leverage Washington has over the isolated, nuclear-armed North just as South Korea and the United States prepare for talks with Kim Jong-un, the North’s leader.

Instead, analysts say, Mr. Trump has been left to exert leverage on South Korea, which is taking the lead in orchestrating the talks, and the South’s president, Moon Jae-in.