President Donald Trump has instructed advisers to prepare a withdrawal from the United States’ free-trade agreement with South Korea, several people close to the process said. The move would stoke economic tensions with the U.S. ally at a time both countries are confronting a crisis over North Korea’s nuclear weapons program.

While Trump could still decide to stay in the agreement to renegotiate its terms, preparations for terminating the deal are far along and the formal withdrawal process could begin as soon as this week, the officials said.

Senior White House officials are trying to prevent Trump from withdrawing from the pact, sources said.

A White House spokeswoman said, “Discussions are ongoing, but we have no announcements at this time.”

South Korea elected a new president, Moon Jae-in, in May, and Trump has been frustrated with Moon’s unwilling to accept U.S. demands, trade experts said. Foreign leaders at first worked hard to build strong relations with Trump, but there has been a change recently, with leaders rejecting his brand of nationalism.

Trump is “playing with fire,” said Gary Schmitt, co-director of the Marilyn Ware Center for Security Studies at the American Enterprise Institute. “There is a new president in South Korea whose instincts probably are to be probably not as pro-America as his predecessor and now you are putting him in situation where he has to react.”

One reason top White House advisers are trying to stop Trump from withdrawing from the South Korea free trade agreement is because they do not want to isolate the government in Seoul at a time when North Korea has become increasingly adversarial.

The trade agreement was signed in 2007 and went into effect in 2012.

Withdrawal could lead to an increase on tariffs levied against products the U.S. imports from South Korea, such as electronics, cellphones and automobiles.