The United States on Wednesday told South Korea it wants to renegotiate a 2012 free trade agreement, citing a lopsided trading relationship.
It is the latest protectionist trade move by US President Donald Trump.
He wants to overhaul the North American Free Trade Agreement and has threatened tariffs on foreign products, such as steel.
He also withdrew from the 12-nation Trans Pacific Partnership agreement in January.
South Korea is a major US trade partner, with the US exchanging about $144.6bn (£112m) in goods and services with the country last year.
The Obama administration had said the 2012 free trade agreement – described at the time as the most significant in 16 years – would boost US businesses.
But exports of American goods to the country have declined almost 3% since the deal went into effect to $42.3bn. At the same time, imports from South Korea have increased by about 23%.
“We can and must do better,” US Trade Representative Robert Lighthizer said in a statement.
The US trade deficit for goods and services with Korea was about $17bn last year, although viewed separately, the US recorded a surplus for services exports.
Wednesday’s letter called South Korea “an important ally and key trading partner” but said the administration “had real concerns about our significant trade imbalance”.
Mr Trump in an interview in April was more blunt, calling it a “horrible deal” and a “one-way street.”
A letter, signed by Mr Lighthizer on Wednesday, asked for a joint session on the subject to occur within 30 days.
South Korean representatives could not be reached for comment.