Panic gripped survivors of Japan's devastating earthquake and tsunami on Saturday after a blast blew the roof off an unstable nuke reactor in Fukushima, north of Tokyo raising fears of a disastrous nuclear meltdown.

Japenese officials declared a nuclear emergency situation at Fukushima I and have issued an alert for Fukushima II after radiation leaked from the nuclear reactor.

Although the government played down fears of radiation leak, the Japanese nuclear agency spokesman Shinji Kinjo acknowledged there were still fears of a meltdown.

Meanwhile, Japan's nuclear safety agency said the nuclear mishap was less serious than both the Three Mile Island accident in 1979 and the 1986 Chernobyl nuclear disaster.

An official at the agency said it has rated the incident at Tokyo Electric Power's nuclear plant north of Tokyo a 4 as per International Nuclear and Radiological Event Scale (INES). Three Mile Island was rated 5 while Chernobyl was rated 7 on the 1 to 7 scale.

Japanese authorities have told the UN's atomic watchdog they are making preparations to distribute iodine to people living near nuclear power plants affected by earthquake, the Vienna-based agency said.

Iodine can be used to help protect against thyroid cancer in the case of radioactive exposure in a nuclear accident.

After the 1986 Chernobyl disaster, thousands of cases of thyroid cancer were reported in children and adolescents who were exposed at the time of the accident. More cases are expected.

Japan's Jiji news agency later said three workers suffered radiation exposure near the Fukushima nuclear plant.

The International Atomic Energy Agency (IAEA) said Japanese authorities had informed it of the explosion and that they were 'assessing the condition of the reactor core'.

Japan expanded the evacuation zone around the plant, Fukushima Daiichi, and also that of the nearby Fukushima Daini nuclear power plant.

'The authorities also say they are making preparations to distribute iodine to residents in the area of both the plants,' the IAEA said in a statement.

'The IAEA has reiterated its offer of technical assistance to Japan, should the government request this,' it said.

Meanwhile, nuclear experts said Japan should not expect a repeat of the Chernobyl nuclear disaster.

According to them, pictures of mist above the plant suggested only small amounts of radiation had been expelled as part of measures to ensure its stability, far from the radioactive clouds that Chernobyl spewed out when it exploded in 1986.

"The explosion at No.1 generating set of the Fukushima nuclear plant in Japan, which took place today, will not be a repetition of the Chernobyl nuclear disaster," said Valeriy Hlyhalo, deputy director of the Chernobyl nuclear safety center.

He was quoted by Interfax news agency as saying Japanese reactors were better protected than Chernobyl, where just over 30 firefighters were killed in the explosion. The world's worst civilian nuclear disaster, Chernobyl has also been blamed for thousands of deaths due to radiation-linked illness.

"Apart from that, these reactors are designed to work at a high seismicity zone, although what has happened is beyond the impact the plants were designed to withstand," Hlyhalo said.

"Therefore, the consequences should not be as serious as after the Chernobyl nuclear disaster."

Japanese officials said on Saturday that the nuclear reactor's core was intact, and that sea water would be poured into the leaking reactor to cool it down and reduce pressure in the unit -- a statement that should calm any fears.

Experts said it was crucial to make sure the steel reactor container had not been shattered in the explosion or in the earthquake.

"If the pressure vessel, which is the thing that actually holds all the nuclear fuel ... if that was to explode -- that's basically what happened at Chernobyl -- you get an enormous release of radioactive material," said Prof. Paddy Regan, nuclear physicist from Britain's Surrey University.

"It doesn't look from the television pictures as though it's the vessel itself."

Television footage showed an explosion in a large building in the area of the number one reactor at the Daiichi nuclear facility. Grey smoke billowed from the site and later, a building was shown without its exterior walls.

Robert Grimes, professor of materials physics at Imperial College London, said earlier it had seemed that back-up generators had failed and had allowed pressure to build up.

"It does seem as if the back-up generators although they started initially to work, then failed," Grimes told BBC television, adding that the explosion was probably the large release of that pressure.

In another development, Japan has asked Russia to increase energy supplies after the nuclear power station damaged, Russian agencies quoted Deputy Prime Minister Igor Sechin as saying on Saturday.

Sechin said Russia can increase liquefied natural gas supply by 150,000 tonnes, while Russia's Mechel and SUEK next week will look into possibility of raising coal supply by 3 to 4 million tonnes, Itar Tass agency reported.-Reuters