"We grant the Japanese government 48 hours to withdraw its troops from Iraq or this infidel will meet the same fate as Berg and the other infidels," al-Qaida Organisation of Holy War in Iraq said in an internet video on Tuesday, which was also reported on Aljazeera.
It was referring to American Nick Berg whom Zarqawi's group professed to have beheaded in May.
"One of the elements belonging to the Japanese forces fell captive in the hands of al-Qaida Organisation of Holy War in Iraq.
"It became evident he had gone to Israel and Jordan before Iraq, and we will show the documents after his family and government identify him," said a masked man on the tape.
Prime Minister Junichiro Koizumi said Japanese troops would not withdraw from Iraq, rejecting the group's demand, Jiji Press and Kyodo news agencies said.
The video showed the anxious captive seated in front of three masked men and a black banner bearing the group's name.
Koizumi has rejected the demand
to pull Japanese troops out
"They are asking why the Japanese government sent its troops to Iraq. They want the Japanese government and Prime Minister Koizumi to withdraw Japanese troops from Iraq or they will cut my head [off]," said the captive who was wearing a white shirt. He spoke in English and then in Japanese.
Japanese foreign ministry spokesman Hatsuhisa Takashima said it was still trying to confirm the information: "Unfortunately, there is no independent report from our side."
Japan's Kyodo news agency quoted the defence ministry as
saying there had been no report of anything unusual related to Japanese troops stationed at Samawa.
Japanese public broadcaster NHK quoted the vice-defence minister as saying his ministry was checking if anyone from its troops Iraq was missing.