Osaka University Hospital says it has successfully transplanted the heart from a girl under the age of 6 into a boy aged under 10.

The girl was declared brain dead on Sunday. Her liver and other organs were harvested for transplantation after her family gave their consent. That paved the way for the first lung transplant between children aged under 15 following a brain-dead declaration, according to the Japan Organ Transplant Network.

"We succeeded in smoothly transplanting (her heart)," operating surgeon Yoshiki Sawa said at a press conference.