U.S. President Donald Trump said Thursday he may hold up a trade agreement reached this week with South Korea until after a deal is reached with North Korea on denuclearization.

"I may hold it up until after a deal is made with North Korea," Trump said in a speech. "You know why? Because it's a very strong card. And I want to make sure everyone is treated fairly," he added.

Senior U.S. officials have expressed concerns privately that Seoul is the weak link in the U.S.-Japan-South Korean alliance and could be too quick to seal a deal with North Korea.