Japan's emperor has said he is "deeply concerned" about the worsening situation at a stricken nuclear power plant, and called on its people to work together to overcome the country's worst crisis since the second world war.

In a rare public address, Akihito said events unfolding at the Fukushima Daiichi plant were "unpredictable", hours after a surge in radiation levels prompted the withdrawal of 70 workers who have been battling to make the facility safe.

"I am deeply hurt by the grievous situation in the affected areas," Akihito said in his first public appearance since Japan's north-east coast was devastated by a powerful earthquake and tsunami in which more than 10,000 people may have died.

Dressed in a dark suit, the sombre-looking 77-year-old said the disaster was on an "unprecedented scale".

"I hope from the bottom of my heart that the people will, hand in hand, treat each other with compassion and overcome these difficult times."

The country remains in a state of high anxiety over the nuclear emergency, but Akihito's appearance may go some way towards lifting people's spirits.

He is widely respected as a thoughtful, intelligent man and his opinions continue to hold sway more than 65 years after his father, Hirohito, was forced to renounce his divine status after Japan's defeat in the second world war.

As a symbolic figure, Akihito must be careful to refrain from involvement in political issues. His address carefully avoided any comment that could be construed as criticism of Japan's nuclear power industry or the official response to the crisis.

It is unlikely that Akihito and Empress Michiko will travel to the disaster zone for as long as the threat persists of a serious nuclear accident.

The imperial household agency said in a statement that the couple wanted to visit the area hit by the tsunami but felt that the focus should remain on the rescue operation for the time being.

The imperial couple made similar visits in the wake of other disasters, including to the western city of Kobe, where 6,400 people died in an earthquake in January 1995.

Akihito thanked emergency workers and the more than 100 countries that have offered aid and assistance with the rescue and recovery effort. "We don't know the number of victims, but I pray that every single person can be saved," he said.

Akihito voiced hope that the engineers and technicians trying to avert disaster in Fukushima would be successful.

"I am deeply concerned about the nuclear situation because it is unpredictable," he said in remarks broadcast on television. "With the help of those involved I hope things will not get worse."