SEOUL (AFP) - A lawsuit filed by a prominent gay film director and his partner seeking legal status for their same-sex marriage was rejected by a South Korean district court on Wednesday (May 25) in the first case of its kind.

Movie director Kim Jho Gwang Soo and his long-time partner Kim Seung Hwan held a wedding ceremony in Seoul in September 2013 and submitted their marriage registration form to their local authority - only for it to be rejected.

While homosexuality is not illegal in South Korea, same-sex marriage is not recognised and the country remains deeply conservative about matters of sexual identity.

The couple challenged the decision with a lawsuit filed in July last year, but the ruling on Wednesday sided with the local authority.

"Circumstances have changed concerning marriage... but unless there is separate legislation, a same sex union cannot be recognised as marriage under the existing legal system", the Seoul western district court said in a statement.

"Related laws, including the Constitution and civil law, are premised on the notion of a conjugal bond meaning a union involving different sexes," it said.

The two Kims are the most high-profile gay couple in South Korea and the lawsuit they filed over their marriage licence was the first of its kind.

"We regret the court decision... but we did see it coming," Kim Seung Hwan told AFP, adding that the couple intended to appeal against the ruling.

The couple's lawyer Ryu Min Hee said the lawsuit had argued that civil law should be viewed through a "gender-neutral" prism that upheld equal rights provisions in the constitution.

"When in doubt, all laws should be interpreted in a constitutional way," she said.

"We're disappointed but we're not done yet," she added.

Ms Ryu's legal team had always acknowledged that the likelihood of a district court judge declaring same-sex marriage legal was extremely slim.

Gay and transgender South Koreans live largely under the radar in a country where many still regard homosexuality as a foreign phenomenon.

But gay rights campaigners were buoyed by the US Supreme Court ruling in June last year that made same-sex marriage legal throughout the United States.