TOKYO - Japan's Supreme Court on Wednesday found a doctor guilty of murder for allowing a patient to die at the request of his family, but suspended her 18-month prison sentence.

The top court, in its first ruling on a so-called mercy killing, upheld a lower court's suspended jail term for Setsuko Suda, 55, who still practices in a hospital in Yokohama near Tokyo, a court official said.

Suda was found guilty of removing a breathing tube from a 58-year-old male patient whose heart and lungs had stopped functioning. At his family's request, she injected a muscle relaxant to hasten his death.

Japan does not have clear rules on how and when doctors are allowed to discontinue life-sustaining treatment or otherwise assist a terminally ill patient's death.

The court ruled that Suda did not have the right to assist the patient's death, saying she "was not in a position to precisely predict the likelihood of recovery or the patient's life expectancy".

"The act was not considered as a forgivable suspension of treatment," the court ruled, without giving further guidance, according to Jiji Press.

Suda said she did not accept the guilty verdict.

"I don't think it's a crime," she said. "In law it may be necessary to specify the life expectancy, but on the medical front we cannot make prim and stiff decisions."

She said the ambiguity caused concern in the medical profession, telling reporters: "Everybody is worried because we cannot draw the line."

Suda also vowed: "I will continue to be a doctor as long as I have a licence. That's my duty."

Prosecutors are investigating two doctors for removing respirators from seven patients at a hospital in Toyama, central Japan, while investigations have been dropped in similar cases in Hokkaido and other regions.

The health ministry has only advised medics "to obtain written documents from patients and make a decision as a group of doctors" when mercy killing should be considered. Masafumi Takai, secretary-general of the Japan Society for Dying with Dignity, called for clearer guidelines.

"It is an urgent task to introduce a legal framework on death with dignity so that doctors will be fully aware of (when they have) immunity and make decisions more confidently," he said.