By David Brunnstrom and Alexandra Alper
WASHINGTON, Sept 30 (Reuters) - The Republican chair of the congressional committee focused on China is calling on the Trump administration to scrutinize US ally South Korea's warming ties with China, which he said pose "significant risks" to the United States.
In a letter to Secretary of State Marco Rubio, Representative John Moolenaar urged "heightened vigilance" about the relationship, citing deepening leader-level ties, South Korean semiconductor investment
in China, and unfair treatment of American firms in South Korea compared with Chinese rivals.
Moolenaar described South Korea as "a major American ally," but warned of "the potential for frictions to develop into a structural shift in Seoul's alignment" requiring "close monitoring and, as necessary, concerted action."
South Korea's foreign ministry said it could not comment in detail on the contents of the letter, which was sent on Tuesday and has not been previously reported. It said that the US-South Korea alliance was the cornerstone of its foreign policy and that they were advancing the alliance across several areas.
The letter comes after a period of tensions in US-South Korea relations that has included some sharp criticism of Seoul by President Donald Trump.
The two countries have been allies since the 1950-53 Korean War, in which China backed North Korea, but differences have surfaced over trade, China policy and security issues, even as Trump has worked to improve his own relations with Beijing.
It also comes as Seoul has pledged hundreds of billions of dollars in US-based investments.
On Tuesday, people familiar with the discussions told Reuters Trump could announce as soon as Wednesday that his administration intends to use South Korea's strategic investment fund for several major US projects including roughly $54 billion for a planned Alaska LNG facility.
Reuters has also reported that SK Hynix was considering leasing part of Intel's Ohio chip plant and even breaking ground on a new chipmaking facility for its subsidiary Solidigm.
Moolenaar called the pace of leader-level engagement between Seoul and Beijing unprecedented, citing Chinese President Xi Jinping's first visit to South Korea in 11 years in 2025 and South Korean President Lee Jae Myung's visit to China this year, the first by a South Korean president since 2019.
He said the "widening gulf" between South Korea and the US was especially troubling because Seoul had "effectively ruled out" supporting a regional response to any Chinese aggression against Taiwan, citing Lee's 2024 election campaign remark: "What does it matter to us what happens in the Taiwan Strait?"
In an interview with Japan's NHK broadcaster in January, Lee said that it would serve South Korea well to "avoid unnecessary involvement," saying it was in the national interest to respect others' positions.
This week, in a post on X mentioning South Korea's booming trade with China, Lee said: "In diplomacy, South Korea-US alliance is most important, but it is also important to manage relations with China."
Lee met with Trump on the sidelines of the UN General Assembly last week, two days before the US president's meeting with Chinese leader Xi.
On technology, Moolenaar argued that South Korea's semiconductor investments in China could leave key US supply chains vulnerable to Chinese pressure. He pointed to Samsung's chipmaking plant in Xi'an and SK Hynix's operations in Wuxi and Dalian, calling them potential points of leverage for Beijing over allied technology production.
Moolenaar also expressed concern over what he described as more lenient South Korean enforcement toward Chinese e-commerce platforms such as Alibaba's AliExpress and Temu when it came to alleged data privacy, consumer protection, cybersecurity and product safety violations compared with that faced by American firms such as Coupang.
The companies mentioned in the letter did not immediately respond to requests for comment. Coupang declined to comment. South Korea's foreign ministry said its regulatory authorities treat all companies operating in Korea fairly.
Moolenaar called for a State Department briefing on developments in South Korea-China relations by October 31.
(Reporting by David Brunnstrom and Alexandra Alper; Additional reporting by Kyu-seok Shim in Seoul; Editing by Don Durfee and Michael Perry)
