"[Ms Yamamoto] was seriously wounded Monday while covering the clashes at Suleiman al-Halabi which have been going on since yesterday. We took her to hospital where she succumbed to her injuries," Rami Abdel Rahmane, the Observatory's president, told AFP, citing medical officials from the hospital.
"She was very likely hit by a projectile," added Mr Rahmane, who was unable to give further details about the missing journalists.
A video posted by militants on YouTube showed a female body lying in a room. It was presented as the corpse of the Japanese journalist, and said she was killed by militia allied to President Bashar al-Assad's troops.
A prominent injury could be seen on her right arm. Next to her, an Asian-looking man appeared to be asking for medical assistance.
AFP was unable to independently confirm the authenticity of the video, or the information presented in it.
French reporter Gilles Jacquier was killed on January 11 at central Syria's Homs, where American journalist Marie Colvin and French photographer both perished on February 22.
