The US stations 28,500 troops in South Korea to deter North Korean aggression. The troop presence is a legacy of the 1950-53 Korean War, which ended in an armistice, not a peace treaty.

This month the allies held their first round of negotiations to renew the current cost-sharing agreement that expires at the end of the year. Seoul's contribution rose to some 960 billion won ($889 million) this year from 150 billion won in 1991.

Cha said the compromise on trade issues was better than expected, given US President Donald Trump's previous threats to withdraw from the KORUS free trade agreement.

"The deal allows President Trump to make the argument that he has helped protect the US steel and auto industries," he wrote.

"On steel, the US essentially leveraged the unilateral tariff on steel (and South Korea's exemption) to achieve a 30 percent reduction in South Korean exports."

Cha also noted that the revision "represents efforts by both sides to keep the alliance on good footing given the need for policy coordination on the North Korean nuclear issue and the upcoming summits between the two Koreas and US and North Korea."

Trump all but nominated Cha to be his first ambassador to Seoul but withdrew the decision after the Korean-American reportedly voiced reservations about the administration's considerations of a limited military strike on North Korea. (Yonhap)