In a video on its website on Sunday, the Ansar al-Sunnah Army purportedly showed its attack on a group of five foreign workers, killing four and kidnapping the fifth, Japanese citizen Akihiko Saito, 44.
Saito's image, however, could not be confirmed, Japan's Kyodo News agency reported from Cairo on Sunday. The report provided no other details and Japan's Foreign Ministry said it could not immediately confirm it.
Meanwhile, Australia's top Muslim cleric has expressed confidence that an Australian captive remains alive and well and that the group holding him will set new conditions for his release.
Shaikh Taj al-din al-Hilali, the leader of Australia's 300,000 Muslims, has been in Iraq for the past week to help secure the release of Douglas Wood, a 63-year-old Australian who was captured in Baghdad 16 days ago.
"While everyone I meet is being very careful with me, I have been assured that Mr Wood is safe and well and that the deadline [for Australia to withdraw its troops from Iraq] has now been extended indefinitely," al-Hilali was quoted by his spokesman in Australia as saying.
Wood's captors had demanded Australia withdraw its troops from Iraq by last Tuesday or they would kill their prisoner.
They had released two videos of Wood before the deadline, but nothing since.
Al-Hilali's spokesman, Keysar Trad, on Sunday said he had spoken to the Mufti, who was optimistic after making indirect contact with Wood's captors and an appeal for his release in Arabic on Aljazeera television.