Optimism that three Japanese held hostage in Iraq would be quickly released evaporated Monday, as Tokyo's top government spokesman backtracked on an earlier statement and said authorities were no longer confident about their safety.

The Japanese are among a growing number foreign nationals kidnapped by Iraqi insurgents — including a Mississippi man whose fate was also unclear and seven Chinese nationals seized by armed men on Sunday.

Vice President Dick Cheney (search), who was in Tokyo on a weeklong Asia tour, promised Prime Minister Junichiro Koizumi (searchthat the United States would "do everything we can to be of assistance."

The Japanese hostages — two aid workers and a photojournalist — were being held by a previously unknown group calling itself the "Mujahedeen Brigades" (search), which demanded Japan pull its troops out of Iraq within three days or it would burn the three alive.

"At one point we were able to make the judgment from various perspectives that they (the Japanese) were safe, but now that's unconfirmed," Chief Cabinet Secretary Yasuo Fukuda told a news conference.

His comments came in sharp contrast to official remarks the day before indicating the three hostages were about to be freed. Japanese media even reported that the military was getting transport planes ready to bring them home.

The hostages were taken amid a recent spate of kidnappings in Iraq, including the abduction of seven Chinese Sunday. But some foreigners were also being released — insurgents freed a Briton and said they were releasing eight other captives of various nationalities.

The American, Thomas Hamill (search), 43, who works for a U.S. contractor in Iraq, was snatched Friday by gunmen who attacked a fuel convoy he was guarding. His captors threatened to kill him unless U.S. troops ended their assault on the city of Fallujah. The deadline passed Sunday morning with no word on his fate.

On Monday, China appealed to Iraqi authorities to rescue the hostages and urged its citizens to avoid Iraq.

The group entered Iraq from Jordan on Sunday and were taken later in the day in Fallujah, the Foreign Ministry said on its Web site. State television said the hostages, aged 18 to 49, didn't work for China's government or a state company.

For the Japanese hostages' families, the uncertainty was taking its toll.

"The anxiety is overwhelming," said Takashi Imai, the father of the youngest of the three hostages, 18-year-old Naoki Imai. "I know the troops are in Iraq to make a contribution — but so is our son. They can't just let him be killed."

Imai, who graduated from high school last month, is a member of a group trying to raise awareness about the health hazards facing civilians in Iraq from depleted uranium munitions used by U.S. troops. Another hostage, 34-year-old Nahoko Takato, worked with street children in the war-ravaged country. The third hostage is a free-lance photojournalist.

Koizumi has staunchly refused to consider the withdrawal demand, a position lauded by Cheney.

"We wholeheartedly support the position the prime minister has taken with respect to the question of the Japanese hostages," Cheney told reporters Monday.

Despite Japan's refusal to withdraw from Iraq, officials said Sunday night that they had received word the hostages would be released unharmed.

Fukuda on Monday acknowledged the government had no evidence that was true.

"We haven't been able to confirm what kind of situation the three hostages are in," he said.

The crisis has swelled into Koizumi's biggest test since he assumed office three years ago.

Despite a deeply divided public, Koizumi championed the plan to send about 1,000 non-combat troops to help in the reconstruction of Iraq in this country's biggest and most dangerous overseas military operation since World War II.

The troops began arriving in the southeastern Iraq city of Samawah in January to carry out water purification projects and to assist in the rebuilding of schools and other infrastructure.

Though many Japanese appear to understand Koizumi's refusal to be cowed, they are also moved by the emotional pleas of the hostages' families.

The crisis began dramatically when a video was aired on prime time television Thursday night showing the hostages surrounded by masked men with automatic rifles and rocket-propelled grenades.

The first sign of a breakthrough came from the Arabic news channel Al-Jazeera, which reported early Sunday it had received a statement from the kidnappers saying they had decided to free their hostages within 24 hours following mediation by a Sunni Muslim organization, the Islamic Clerics Committee.