South Korea’s largest air carrier Korean Air’s buyout of Asiana Airlines faced yet another hurdle, as the US antitrust regulator decided to conduct a more thorough investigation before granting approval of the merger.

The US Federal Trade Commission was expected to announce its decision this week, but informed Korean Air on Wednesday it would need more time for a thorough evaluation, according to the Koran air carrier.

In March, the US antitrust watchdog evaluated that when merged with Asiana Airlines, Korean Air would virtually monopolize the popular Incheon-Los Angeles route at home and elevated the investigation to its in-depth second-stage review.

At the same time, the FTC requested that Korean Air submit plans on how to ensure competition.

In response, Korean Air in August submitted required documents, including plans on how to share the cited routes with other air carriers to reduce antitrust concerns.

The watchdog had said it would take 75 days to review the case.

“We have been fully engaged in submitting documents and responding to investigations required by the US, and we plan to continue to do so to get its green light for the merger,” said Korean Air in an official statement.

Just a day earlier, British competition authorities had expressed concerns over the merger, citing worries that Korean Air and Asiana Airlines could possibly dominate routes between the two countries.

Britain’s antitrust regulator asked Korean Air to submit remedies to address its concerns by Nov. 21.

Korean Air’s merger deal still awaits approval from the EU, the UK, the US, Japan and China, after receiving the green light from nine other countries since January 2021, when the company submitted documents to antitrust regulators in 14 countries.

In November 2020, Korean Air inked a deal to buy a 63.88 percent stake in its longtime rival Asiana Airlines for 1.8 trillion won ($1.36 billion).

Korean Air, the world’s 18th-largest carrier by fleet, is expected to rise to No. 10 if the merger deal closes as planned.