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SEOUL, Sept 25 (Reuters) - South Korean President Lee Jae Myung told U.S. Treasury Secretary Scott Bessent that trade talks with the United States should be "commercially rational" and meet the interests of both countries, the president's office said on Thursday.

Lee spoke to Bessent at the United Nations on the sidelines of the General Assembly on Wednesday, his chief secretary for policy, Kim Yong-beom, told a briefing in New York.

The meeting focused on the $350 billion package of investment from South Korea agreed in principle between Lee and U.S. President Donald Trump at a summit in July as part of a deal to lower tariffs against South Korean goods, Kim said.

"With regard to the investment package with the U.S., (Lee) expressed hope that the discussions would progress based on commercial rationality and in a direction that serves the interests of both countries," Kim said.

He said South Korea’s economy and its foreign exchange market — which differ significantly from Japan’s — should be key factors in the ongoing talks on a final agreement.

Finance Minister Koo Yun-cheol, who was also with Lee, separately held discussions with Bessent to discuss the U.S. investment package and a currency swap but the ministry declined to elaborate on details.

"It would be wise to interpret it (request of a currency swap) as a strategy to avoid a direct investment of a large scale, because Korea doesn't have the money," Kim Yong-jin, management professor at Sogang University in Seoul, said. "Unless there is some unlimited dollar liquidity guarantee in place, $350 billion sounds like an impossible number."

Japan formalised a trade deal with the U.S. earlier in September to lower tariffs on its exports. The agreement includes Japan investing $550 billion in U.S. projects.

South Korea's Lee has said that a similar arrangement involving large capital outflow to the United States could destabilise the currency market and drain South Korea's foreign reserves.

The won =KRW broke the psychologically important level of 1,400 per dollar overnight and closed at 1,403.8 early on Thursday, the weakest level since mid-May.

South Korea is seeking a foreign exchange swap with an unlimited credit line from the U.S. to support any final trade agreement, Kim said. South Korean officials have said Washington is reviewing the FX swap proposal.

(Reporting by Jack Kim, Hyunjoo Jin, Cynthia Kim and Jihoon Lee; Editing by Leslie Adler and Lisa Shumaker)

((jack.kim@thomsonreuters.com; +822 6936 1455;))