Two South Korean television stations said Friday morning local time that the South Koreans -- Christian missionaries -- were freed, but the Foreign Ministry in Seoul could not confirm the report, according to the Associated Press.
Japan's government said it does not intend to pull out of Iraq.
The kidnappings mark a possible new tactic by militants to try to pressure governments allied with the United States.
The Arab network Al-Jazeera aired portions of a video released by a previously unknown group calling itself the Mujahedeen Squadrons. The video showed two Japanese men and a woman blindfolded and surrounded by gunmen dressed in black who display the captives' passports.
An Al-Jazeera announcer read a statement he said came with the video in which the kidnappers issue a three-day deadline for Japan to announce it is withdrawing its troops. "Three of your sons have fallen into our hands," the Al-Jazeera announcer read. "We offer you two choices: either pull out your forces, or we will burn them alive. We give you three days starting the day this tape is broadcast."
According to the Arab network, the three were taken in southern Iraq, where about 530 Japanese troops out of a 1,100-member planned deployment are based and where Shiite militiamen have been engaged in an uprising this week.
Japan's NHK television identified the captives as two aid workers and a journalist.
The eight South Koreans, believed to be evangelists from the Christian Council of Korea who set out for Iraq on April 5, were detained by unidentified armed men, a Foreign Ministry official told the AP.
One escaped and the seven others were released, according to the SBS and YTN TV news stations.
Israeli and Iranian media, meanwhile, reported that two Arab residents of Jerusalem were kidnapped by the Ansar a-Din group, which accused them of spying for Israel.
"We have captured spies who belong to the Zionist enemy, and we demand the immediate release of all the prisoners who belong to the religious factions (in Iraq), especially the women," a masked man said in the tape, according to Reuters. "We will negotiate over these two captives."
The video showed the frightened-looking abductees confessing to being spies. One said he worked for RTI International, a North Carolina research institute under contract with the U.S. Agency for International Development to participate in reconstruction in Iraq.