TOKYO - Optimism that three Japanese held hostage in Iraq would be quickly released evaporated today, as Tokyo's top government spokesman now suggested authorities were no longer confident about their safety.

The Japanese are among a growing number of 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, who was in Tokyo on a weeklong Asia tour, promised Prime Minister Junichiro Koizumi that 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," 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. 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, 43, a truck driver for a U.S. contractor in Iraq, was snatched Friday by gunmen who attacked a fuel convoy. 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.

China today appealed to Iraqi authorities to rescue its citizens taken hostage and urged Chinese to avoid Iraq.

The seven Chinese entered Iraq from Jordan on Sunday and were taken later in the day in Fallujah, China's foreign ministry said. State television said the hostages, aged 18 to 49, didn't work for China's government or a state company.

It wasn't clear why the seven Chinese were in Iraq.