The emergency at Japan's wrecked nuclear plant appeared to be worsening today.

Surging radiation levels forced engineers to temporarily withdraw, losing time in a desperate operation to cool the overheating reactors.

The technicians were dousing the nuclear reactors with seawater in an effort to cool them when they had to retreat in the late morning.

They returned in the evening after radiation levels subsided, but in the hours between, it was not clear what if any operations continued.

Once again official information on what was happening at the Fukushima Dai-ichi plant was scarce.

White steam-like clouds drifted up from one reactor which, the government said, probably produced the burst of radiation that led to the workers' withdrawal. The plant's operator reported a fire at another reactor for the second time in two days.

"The anxiety and anger being felt by people in Fukushima have reached boiling point," the governor of Fukushima prefecture, Yuhei Sato, said. He criticised preparations for an evacuation if conditions worsen and said centres already housing people moved from nearby the plant do not have enough hot meals and basic necessities.

The nuclear crisis has triggered international alarm and partly overshadowed the human tragedy caused by Friday's earthquake and the subsequent tsunami, a blast of black seawater that pulverised Japan's north-eastern coastline. The quake was one of the strongest recorded in history.

Millions of people struggled for a fifth day with little food, water or heat, and already chilly temperatures turned to snow in many areas. Police say more than 452,000 people are staying in temporary shelters, often sleeping on the floor in school gymnasiums.

Nearly 3,700 people are officially listed as dead, but it is feared the toll will climb over 10,000 since several thousand more are listed as missing.

In a rare address to the nation, Emperor Akihito expressed condolences and urged Japan not to give up.

"It is important that each of us shares the difficult days that lie ahead," said Akihito, 77, a figure deeply respected across the country. "I pray that we will all take care of each other and overcome this tragedy."

He also expressed his worries over the nuclear crisis, saying: "With the help of those involved I hope things will not get worse."

Since the quake and wave hit, authorities have been struggling to avert an environmental catastrophe at the Fukushima Dai-ichi complex, 140 miles north of Tokyo. The tsunami destroyed the backup diesel generators needed to keep nuclear fuel cool at the plant's six reactors, setting off the atomic crisis.

In the city of Fukushima, about 40 miles inland from the nuclear complex, hundreds of harried government workers, police officers and others struggled to stay on top of the situation in a makeshift command centre.

An entire floor of one of the prefecture's office buildings had been taken over by people tracking evacuations, power needs, death tolls and food supplies.

In one room, uniformed soldiers evaluated radiation readings on maps posted across a wall. In another, senior officials were in meetings throughout the day, while nuclear power industry representatives held impromptu briefings before rows of media cameras.

Today's radiation surge was believed to have come from Unit 3, where workers are struggling with a fuel storage pond believed to be leaking radiation, as well as possible damage to the containment vessel - the thick concrete armour built around the reactor - that would allow radiation to escape.

Elevated levels of radiation were detected well outside the 20-mile emergency area around the plants. In Ibaraki prefecture, just south of Fukushima, they were about 300 times normal by late morning.

A little radiation was also detected in Tokyo, triggering panic buying of food and water.

John Price, a nuclear safety expert, said he was surprised by how little information the Japanese were sharing.

"We don't know even the fundamentals of what's happening, what's wrong, what isn't working. We're all guessing," he said. "I would have thought they would put on a panel of experts every two hours."

There are six reactors at the plant. Units 1, 2 and 3, which were operating last week, shut down automatically when the quake hit. Since then, all three have been rocked by explosions.

Compounding the problems a fire broke out yesterday in Unit 4's fuel storage pond, an area where used nuclear fuel is kept cool, causing radioactivity to be released into the atmosphere.

Units 4, 5 and 6 were shut at the time of the quake, but even off-line reactors have nuclear fuel - either inside the reactors or in storage ponds - that need to be kept cool.