Aljazeera television on Thursday aired a silent video of the three Japanese - two men and a woman - and read out a statement addressed to "the friendly people of Japan" from an Iraqi group called Saraya al-Mujahidin.
The statement gave Tokyo three days to withdraw its forces from Iraq or have its citizens executed.
"We are the sons of Muslim Iraqi people. We were friends and on good terms. Why did you betray us and support the US forces which have violated our soil, our sacred places and land and shed the blood of our children?" demanded the statement.
"It is time to retaliate. Three of your nationals are now in our custody and you have either to withdraw, or we will burn them alive."
Noriaki Imai is an aid worker who
arrived in Iraq one week ago
A Japanese government spokesman said Tokyo had no plans to withdraw its troops and demanded the immediate release of its citizens.
Japan's NHK television identified them as two journalists and an aid worker. One of the hostages being held by Saraya al-Mujahidin is a woman.
The captives were named as as Noriaki Imai, Soichiro Koriyama and Nahoko Takato. Imai, an aid worker, had left for Iraq on 1 April, NHK reported.
Japanese Prime Minister Junichiro Koizumi has been one of the strongest backers of the US-led invasion of Iraq, a stance that has raised concern over Japanese troops being targeted.
Japan has sent 550 troops to the southern town of Samawa on a non-combat mission. It is Tokyo's riskiest military deployment since the second world war and critics say it violates the country's pacifist constitution.
No Japanese soldier has fired a shot in action or been killed
in combat since 1945 and casualties could undermine
support for PM Koizumi's government ahead of Upper House elections in July.
Meanwhile, seven South Koreans who were captured near Baghdad on Thursday, have been released.
US ally South Korea has 600 army
personnel stationed in Iraq
The missionaries, all members of an evangelical South Korean Presbyterian church, were travelling in two cars from Jordan to Baghdad when they were seized on Thursday by a group of 25 to 30 armed men on a road just north of the capital.
"At first we were scared and afraid of what would happen. I felt a major gap between our culture and theirs," Lim Young-sup, a church minister, told Reuters.
A total of eight missionaries were travelling in the cars but one managed to escape, the others said. The remaining seven were blindfolded and taken to a house by the kidnappers, who
initially accused them of being members of the CIA.
Hong Kwang-chun said the missionaries tried to tell the Iraqis they were NGO workers and meant no harm.
"One of us knew some medicine and so we got our medical kit and showed them that we were doctors and nurses," he said. "Then their attitude changed completely and they became very friendly. They gave us food and something to drink."
They were held for around five hours, the Koreans said, before being driven back to the outskirts of the capital escorted by their abductors. They were left with all their possessions apart
from $30,000 in cash they had brought into Iraq.
US-ally South Korea has 600 military engineers and medics
in Iraq and plans to send 3000 more for reconstruction.
The incident took place after two South Koreans were released on Tuesday after they were detained for 14 hours by Shia leader Muqtada al-Sadr's militiamen.
Two Arabs have also been captured in Iraq, reported Israeli media on Thursday.
In footage from Iranian television that was rebroadcast on Israeli television, the men identified themselves as Nabil Razuk, 30, and Ahmad Yasin Tikati, 33. They identified themselves as aid workers.
Iraq is becoming increasingly
dangerous for foreigners
Antoine Razuk said his nephew, who has an Israeli citizenship and lives in occupied East Jerusalem, was working with the US Agency for International Development (USAID).
It was unclear whether Tikati also has Israeli citizenship. The Israeli Mossad security service is overseeing the investigation into the report, reported Israeli security sources.
Meanwhile, a British civilian was kidnapped this week in the southern Iraqi city of Nasiriya, the scene of heavy fighting between al-Sadr militiamen and Italian troops, said an occupation official on Thursday.
The official named the man as Gary Teeley, a British contractor. British media said Teeley, 37, was said to be a resident in the Middle East and had been working at a US airbase.
A Foreign Office official in London confirmed that Teeley
was missing, but would not say what he was doing in Iraq or
comment on the manner of his disappearance.
Separately, a Canadian man working for a US humanitarian organisation has been taken hostage in Iraq, a foreign ministry spokesman said Thursday.