A senior House Republican wants Washington to take a harder look at Seoul's warming relationship with Beijing, a negative signal for the US-South Korea alliance and for Taiwan.

• John Moolenaar, who chairs the House committee on strategic competition with the Chinese Communist Party, wrote to Secretary of State Marco Rubio calling the trend a major risk.
• His concerns: more top-level contact between Seoul and Beijing, South Korean chipmakers investing in China, and US companies getting worse treatment in South Korea than Chinese rivals.
• He warned that friction could push Seoul into a lasting shift away from the US, and called for closer watch and coordinated action.
• The sharpest worry is Taiwan: Seoul has effectively ruled out backing a regional response if China attacks, and President Lee Jae-myung has said South Korea should avoid unnecessary involvement.
• South Korea's foreign ministry said it could not comment in detail on the letter, but called the US alliance the foundation of its foreign policy.

Outlook: Expect more pressure from Congress on Seoul over its China and Taiwan positions, even as Trump keeps trying to improve his own relationship with Beijing.