Nuclear safety officials in Japan fear the core of a reactor at the Fukushima Daiichi nuclear power plant may have cracked, causing a leak of high levels of radiation.

Growing uncertainty over the state of the stricken reactor prompted the government to tell people living within a 12-19 mile (20-30km) radius of the plant to consider leaving their homes temporarily.

The government's chief spokesman, Yukio Edano, said 130,000 residents in the area had been encouraged to leave to improve their quality of life, not because their health was at risk.

The nuclear emergency, 150 miles north of Tokyo, has caused severe disruption to business, supply routes and other services in the area.

On Thursday, three workers were exposed to radiation after stepping in contaminated water in the turbine building of the No 3 reactor. They were trying to cool the crippled reactor when the accident occurred.

"The contaminated water had 10,000 times the amount of radiation as would be found in water circulating from a normally operating reactor," said Hidehiko Nishiyama, a spokesman for Japan's nuclear safety agency. "It is possible that there is damage to the reactor."

Two of the men received possible beta ray burns to their legs. All three have been transferred to a special radiation treatment facility.

Edano said the source of the leak remained unknown. "We are exploring every possibility, but we don't think this is a new situation, rather that a certain amount of radiation may have leaked from the reactor. This is a possibility that we have been mentioning for some time.

"But at this point we don't know if the radiation is coming from the reactor itself or from another source."

Nuclear officials say the leak may have come from pipes or the reactor's pool for storing spent fuel rods, which workers have been struggling to cool off since the plant was badly damaged in the 11 March earthquake and tsunami.

Officials were preparing themselves for the possibility that the reactor core was damaged in an explosion three days after the disaster that destroyed its containment building. The reactor contains 170 tonnes of radioactive fuel in its core, and is the only one of the facility's six reactors that contains the potentially more dangerous plutonium-uranium mixed oxide fuel.

Thursday's accident forced a temporary halt to work on two reactors while technicians check radiation levels.

"We should try to avoid delays if at all possible, but we also need to ensure that the people working there are safe," Nishiyama said.