A video released by the Islamic State purports to show the group threatening to kill the two Japanese hostages. (Associated Press)
The apparent murder of a Japanese hostage by Islamic State was a reminder of the group's 'murderous barbarity', British Prime Minister David Cameron said on Saturday, as Japan scrambled to verify an online video.
"The reported brutal murder of Haruna Yukawa and the further threats made by ISIL are yet another reminder of the murderous barbarity of these terrorists," Mr Cameron said in a statement issued by his Downing Street office.
Images of fellow Japanese hostage Kenji Goto holding a photo apparently showing that Mr Yukawa has been murdered have been posted online, but several supporters of the IS group on social media channels have contested the veracity of the video.
Prime Minister Cameron offered his "thoughts and prayers" to Mr Yukawa's family and said that Britain "stands in solidarity with the Japanese people at this difficult time."
He also backed Japanese counterpart Shinzo Abe's "firm stance" in dealing with IS, which demanded a $200 million ransom for the hostages' release.