Late on Friday, the Japanese news agency Kyodo said a website reportedly operated by the Sunni group Ansar al-Sunnah Army claimed that Japanese hostage Akihiko Saito had died.
Foreign Ministry spokesman Akira Chiba said on Saturday that the ministry could not tell whether the image showed Saito, a 44-year-old security consultant who has been missing in Iraq since his convoy was ambushed early in May.
The internet video showed identification papers and a passport of Saito, a former paratrooper and veteran of the French Foreign Legion, who had been missing in Iraq since 8 May.
It displayed the body of a dead man on his back with a bloodied face that resembled Saito's pictures.
Saito worked for Hart Security Limited, a British security firm based in Cyprus.
"We're not able to confirm anything at this point," Chiba told The Associated Press.
Saito was among a group of five foreign workers - four of them confirmed dead - who were ambushed in the vast al-Anbar province west of Baghdad. Tokyo has been unable to independently confirm Saito's whereabouts or condition.
Saito's younger brother, Hironobu, said he had not seen the pictures but had been in contact with government officials.
"Please don't view my brother as a representative of Japan in Iraq"
"The Foreign Ministry has said it's fairly certain" the pictures are of Saito, he told reporters, in comments aired by public broadcaster NHK. "We believe there's a 90% chance that he is dead ... We probably won't be able to contact them (his captors)."
"I have been mentally preparing myself" for the possibility of his death, he said.
Japan has dispatched more than 500 troops on a humanitarian mission to the southern Iraqi city of Samawah, but many Japanese have criticised the military deployment as a violation of Japan's pacifist constitution.
"Please don't view my brother as a representative of Japan in Iraq," Hironobu Saito said.
Unlikely to survive
An unidentified man who worked with Saito at Hart in Baghdad and who survived the ambush also told Japanese officials this month that Saito was bleeding massively after being shot and it was unlikely that he could have survived.
Saito served in the French Foreign Legion for 21 years, but legionnaires said they could not help free him because he was not a French citizen.
Japanese soldiers are in Iraq on
a humanitarian mission
Last year, five Japanese were taken hostage in Iraq and later released, and many Japanese criticised them for rashly endangering themselves.
In October, Iraqi fighters beheaded a Japanese backpacker, and many there blamed the victim for travelling to the war-torn country despite safety warnings.
However, those incidents stirred opposition to Japan's non-combat deployment to Iraq.