Earlier on Sunday a self-styled "Iraqi mediator" told Aljazeera the captors would start killing the Japanese hostages starting on Monday if Tokyo did not begin withdrawing its troops.
But other sources in Iraq later said that Mizhir al-Dalaimi, the head of the League for the Defence of Iraqis' Rights, was playing no part in negotiations to secure the civilians' release.
Aljazeera has also received a letter from the families of the Japanese hostages, pleading for information and backing their captors' demand for Tokyo to pull its troops out of the war-ravaged country.
In the letter, which was read out on Aljazeera television, the families "assert that the Japanese people demand an immediate withdrawal of Japanese forces from Iraq". The families also asked Aljazeera to obtain film footage showing their relatives were still alive.
The hostages' families have also handed their government a petition with what they said were 150,000 signatures, urging it to withdraw Japanese troops if it would help.
Meanwhile, Japan has put three C-130 transport planes on standby in Kuwait so it could use them to airlift the three Japanese hostages, a Japanese Defence Ministry source said on Monday.
On Thursday, Aljazeera aired a videotape showing the three Japanese sitting at the feet of their armed captors. Japan was stunned and Prime Minister Junichiro Koizumi was confronted with the biggest test of his political career.
The captors' are also demanding that Japanese deputy foreign minister should visit Falluja, a city west of Baghdad which US forces this week placed under siege, "to see the massacres and mass graves committed by US forces", said al-Dalaimi.
Soichiro Koriyama is a former
soldier turned photojournalist
The hostage takers want Tokyo "to apologise to the people of Iraq," he added.
When asked by Aljazeera who asked asked him to mediate and pass on the kidnappers' demands, al-Dalaimi said "the leadership of the resistance".
The hostages are Noriaki Imai, 18, Soichiro Koriyama, 32, and Nahoko Tokato, 34.
Imai graduated from high school last month. He is a member of the Campaign to Abolish Depleted Uranium and travelled to Iraq on 1 April to study the effects of depleted uranium on Iraqi children.
Noriaki Imai planned to study the
effects of depleted uranium
Koriyama is a former soldier turned freelance photojournalist.
Takato, is an aid worker and peace activist. She travelled to Iraq in April 2003, after US and British tanks entered Baghdad.
The captors had initially threatened to burn the hostages alive if Tokyo did not withdraw its troops from Iraq.
A previously unknown group called Saraya al-Mujahidiin sent a fax to Aljazeera saying it would release all three hostages by 00:00 GMT (3:00am), adding they were all in good health.
Earlier, the group said it was releasing the hostages after the Association of Muslim Scholars (AMS) - the highest Sunni Muslim authority in Iraq - called for the captives release.
AMS spokesman Muhammad Bashar al-Faiyad reiterated calls for the hostages to be freed after the deadline for their release was reached on Sunday.
Al-Faiyad also urged the kidnappers to stick to their promise to release the civilians. He called on all kidnappers in Iraq to release their hostages, in an interview with Aljazeera television.
Other foreigners caught up in the recent wave of kidnappings include a presumed American citizen, a Canadian aid worker and a group of 30 foreigners, all been reportedly abducted in Iraq in recent days.
Germany said that two of its missing nationals were probably dead.
But a British person and a group of Asian drivers were released on Sunday.
And within hours of their release, China's official media reported that seven Chinese citizens had been kidnapped by armed men in central Iraq on Sunday.
The Xinhua news agency quoted a Chinese diplomat in Baghdad as saying the group entered Iraq via Jordan early on Sunday and were most probably abducted in the flashpoint city of Falluja, west of Baghdad. They were later released.
China, a permanent member of the UN Security Council, resolutely opposed the invasion of Iraq and has refused to send troops to help police the US-led occupation.