Firefighters found Byun Hee-soo in her home in Cheongju after a mental health counsellor called emergency services to report that she had not been heard from for several days, Yonhap news agency reported.

South Korea remains deeply conservative about matters of sexual identity and is less tolerant of LGBT rights than some other parts of Asia, with many gay and transgender Koreans living largely under the radar.

Byun, formerly a staff sergeant and in her 20s, enlisted voluntarily in 2017. She went on to have gender-reassignment surgery in 2019 in Thailand.

The defence ministry classified the removal of her male genitals as a mental or physical handicap, and a military panel ruled last year that she would be compulsorily discharged.

At the time she waived her anonymity to appear at a press conference to plead to be allowed to serve, wearing her fatigues and saluting the gathered journalists and cameras.

"I'm a soldier of the Republic of Korea," she said, her voice breaking.

Police confirmed her death to AFP and said they were investigating.

Reports said no note was found but the death was being treated as suicide, with Yonhap citing officials saying she had tried to kill herself three months ago.

Byun's death triggered an outpouring of grief and calls for South Korean MPs to pass an anti-discrimination bill.

"The whole of Korean society bears responsibility for her death," said a poster on Daum, the country's second-largest portal.

"Those who ridiculed her and made malicious online comments because she was transgender, I want you to reflect on what you did to her."

- Childhood dream -

South Korea has a conscript army to defend itself against the nuclear-armed North, with all able-bodied male citizens obliged to serve for nearly two years.

But Byun was a volunteer non-commissioned officer and said at her press conference last year that serving in the military had always been her childhood dream.

"Putting aside my sexual identity, I want to show everyone that I can be one of the great soldiers defending this country," she continued, fighting back tears. "Please give me that chance."

Her case was the first of its kind in South Korea.

Deputy defence ministry spokesman Moon Hong-sik expressed condolences over what he called "the regrettable death of the late former staff sergeant Byun Hee-soo".

But he added there had been no detailed discussions about transgender soldiers serving in the military.

International rights groups have previously voiced concern about the way the country treats gay soldiers, who are banned from engaging in same-sex acts and can face up to two years in prison if caught -- even though such actions are legal in civilian life.

Seo Ji-hyun, a prosecutor who triggered the country's #MeToo movement by going public over sexual harassment she suffered at the hands of her superior, declared following Byun's death: "We could have saved her... We just had to let her live life true to who she was."

"Right now anti-discrimination bill", she added as a hashtag on her Facebook account.

A new bill was proposed last year to take on the country's deep-seated traditional social values, which are reinforced by powerful megachurches that condemn homosexuality.

The measure would ban favouritism based on sex, race, age, sexual orientation, disability or religion as well as several more unusual criteria such as criminal history, appearance and academic background.

More than a dozen attempts to pass broad anti-discrimination laws have failed over the past 14 years in the face of strong opposition from conservative churches and civic groups.