The headless body of a man found in Iraq has been confirmed as that of Japanese hostage Shosei Koda.
Koda pleaded for government help in a televised video last week
Prime Minister Junichiro Koizumi condemned the killing as a "cruel and inhuman act" and said Japan would "resolutely fight against terrorism".
Meanwhile, Arabic TV has shown footage of a Sudanese hostage, and in another televised videotape a Polish-Iraqi woman has pleaded for her life.
They are among dozens of foreign hostages currently held in Iraq.
Hostage-takers often ascribe political motives to kidnapping foreigners, whereas ransoms have usually been demanded for the hundreds of Iraqis who have been taken.
The Islamist kidnappers of backpacker Shosei Koda gave a 48-hour deadline for the withdrawal of all Japanese troops when they took him on Wednesday.
A decapitated body found on Friday and thought to be that of Koda turned out to be that of an Iraqi, about whom no more details were given.
Then news agencies were shown a video depicting a corpse in a white, bloodstained shirt next to the severed head of an East Asian-looking man with a thin beard.
On Sunday, Japanese Foreign Minister Nobutaka Machimura said the body had been found in a field in an insurgent-controlled part of Baghdad, and was that of Koda.
Koda's kidnappers are thought to have been the al-Qaeda in Iraq group led by the Jordanian militant Abu Musab al-Zarqawi.
Mr Koizumi said his country would stay in Iraq "for the sake of the people of Iraq and continue resolutely to fight against terrorism", according to French news agency AFP.
Troops to stay
Tokyo has repeatedly refused demands to pull its 550 troops out of Samawa, southern Iraq. The troops are supposed to be undertaking humanitarian work but are said to be mostly confined to their camp because of security fears.
Correspondents say the beheading could badly affect Prime Minister Koizumi, who is a strong ally of the Bush administration and sent troops to Iraq in the face of public opposition.
However, some add that many in Japan feel Koda ignored firm government advice to stay away from Iraq.
Opposition politician Katsuya Okada said the beheading "never would have happened" had Japan not sent troops to Iraq.
"I want to strongly request a withdrawal," AFP reported Mr Okada as saying.
Koda, a 24-year-old from Nogata in Japan's Fukuoka prefecture, left on a year-long trip in January, starting in New Zealand.
Another hostage, Polish-born Teresa Borcz-Khalifa, pleaded for her life
He told people he met on the way that he wanted to go to Iraq out of curiosity.
Officials said he took a bus to Baghdad from Amman in Jordan last week, and was seen up until last Sunday attempting in vain to find accommodation in the city.
Earlier this year, Japan managed to free five other citizens taken hostage in Iraq after days of mediation. Koda is the first Japanese national to be killed by hostage-takers.
In other developments:
- Al-Arabiya TV showed a video tape of a Sudanese man identifying himself as Noureddin Zakaria, an interpreter who said he had been kidnapped and called on his US employer Titan Corp to leave Iraq
- Al-Jazeera TV showed a video tape of Polish-Iraqi hostage Teresa Borcz-Khalifa pleading for her life, but French news agency AFP said the Polish government had immediately rebuffed a demand that it withdraw its 2,500 troops in Iraq
- Two truck drivers working for a Kuwaiti transport company, Bangladeshi national Abul Kashem and Sri Lankan national Dinesh Dharmendra Rajaratnam, were confirmed as kidnapped
- There was no fresh news on the fate of British-Iraqi aid worker Margaret Hassan, who was seized 12 days ago on her way to work.