Trump offered no explanation for the apparent threat, which came just a day after he hailed the US-Korea Free Trade Agreement (KORUS) as a victory for his get-tough stance on commerce.
"I may hold it up until after a deal is made with North Korea, does everybody understand that? You know why? Because it's a very strong card," Trump said in a speech in Ohio.
"South Korea has been wonderful, but we'll probably hold that deal up for a little while, see how it plays out."
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On Wednesday, Trump had seemed to be firmly behind the renegotiated trade deal, which should now give US carmakers greater access to South Korean markets.
"@USTradeRep just announced an agreement in principle with South Korea on KORUS!" he said.
"A great deal for American and Korean workers. Let's now focus on our important security relationship."
But the president's remarks in his Ohio speech appeared to suggest he is seeking to hold the deal as leverage over Washington's ally South Korea ahead of the nuclear talks.
North Korea and South Korea plan to hold a summit on April 27.
Trump is then due to meet North Korea's Kim Jong Un before the end of May for talks on Pyongyang's denuclearization.
