Delta Air Lines has been accused of firing four employees for speaking Korean in the workplace.

A group of former Delta employees Ji-Won Kim, Lilian Park, Jean Yi and Jongjin An, all of whom are of Korean descent, claim they were fired for speaking Korean by Delta in May 2017, KIRO 7, a local channel in Seattle, Washington, reported on Tuesday.

An was told by her manager that there had been complaints from non-Korean agents that “they feel uncomfortable” and was told to “limit speaking Korean.”

Yi told the outlet that Korean-speaking passengers were so glad to see her and her speaking Korean made them feel comfortable as they did not speak English.

The four women, who have filed a lawsuit against the US airline, believe their fluency in Korean was one of the key reasons why they were hired by Delta as the company has flights between Sea-Tac Airport in Seattle and South Korea.

The details of the lawsuit filed in King County Superior Court allege that “though assigned to work flights to and from Korea, composed of many Korean-speaking passengers, they were singled out and admonished for speaking Korean,” according to KIRO 7.

The lawsuit also shows the women, three of whom are US citizens, “were suspended and ultimately terminated for allegedly offering unauthorized upgrades.”

The former Delta employees argued, however, that offering free upgrades on an oversold flight is a pretty standard practice and that other agents do it on a daily basis.

The women have reported sexual harassment in the workplace by the same Delta employee in the past also, but the agent reportedly still works at the company.

“We also suspect that their termination is related to the reporting of sexual harassment,” Jennifer Song, the attorney representing the women, told KIRO 7.

By Yim Hyun-su (hyunsu@heraldcorp.com)