"We have a bad deal with South Korea. We're just starting negotiations with South Korea. South Korea, we protect, but we're losing $40 billion a year with South Korea on trade," he added.

"It's a horrible deal. So we're starting -- we started, as of yesterday, renegotiating the deal with South Korea. We have to."

South Korea and the US enforced their free trade deal also known as KORUS FTA in 2012, intended to remove or lower trade barriers to markets. South Korea sees the deal as mutually beneficial by helping expand bilateral trade and exchanges in diverse areas.

Trump, however, has blamed free trade deals including the one with South Korea for killing US jobs.

On Wednesday (local time), US Trade Representative formally notified South Korea that it wants to start the process of revising the free trade agreement between the two countries.

"When the KORUS Agreement was negotiated, expectations were high that both of our economies would realize significant gains,"  USTR Robert Lighthizer said in his letter to South Korea's commerce ministry in charge of trade issues.

"However, our overall deficit with Korea has increased, and our goods deficit has doubled since the Agreement entered into force...

It is critical that we achieve real progress that fosters a truly fair and level playing field, and a more balanced trade relationship," he added. (Yonhap)