The presidential office has pledged to continue trade and tariff negotiations with the United States while placing top priority on national interests, after U.S. Secretary of Commerce Howard Lutnick ramped up pressure by saying there would be no flexibility.

An official at the top office said on Friday that negotiations will not go against rationality or fairness, in line with the position stated by President Lee Jae Myung the previous day during his 100-day press conference.

Lutnick said in an interview with CNBC on Thursday that "there is no flexibility" in the framework trade deal struck with South Korea in July, adding that Seoul should either accept it, or pay the tariffs.

Lutnick said Lee and U.S. President Donald Trump did not talk about the deal when they held a summit at the White House in late August because the South Korean leader did not sign the paper, adding "the devil is in the details."

Seoul and Washington have yet to narrow differences on the terms of South Korea's investment pledge, profit sharing and other details.