A Japanese nuclear power plant remains in crisis following Friday's massive earthquake and tsunami, with an explosion at a second reactor and cooling functions failing at a third.

The plant's operator, Tokyo Electric Power (TEPCO), says the cooling system at the No. 2 reactor at its Fukushima plant has now failed, Jiji Press reported.

The No. 1 and No. 3 reactors experienced the same problem when their cooling systems were knocked out by the 8.9-magnitude quake and subsequent tsunami which struck a large swathe of the country's coast north of Tokyo.

The failures were followed by explosions in the buildings surrounding those reactors.

Today's hydrogen explosion in the No. 3 reactor complex sent a plume of smoke billowing into the sky.

The UN atomic watchdog has released a statement saying the No. 3 reactor container was not breached during the latest explosion and there was no major rise in radiation.

"Radiation dose rate measurements observed at four locations around the plant's perimeter over a 16-hour period on 13 March were all normal," the IAEA said.

"The reactor building exploded but the primary containment vessel was not damaged. The control room of [No. 3] remains operational."

TEPCO says nine people were injured in the latest explosion.

"I was having lunch at a restaurant when I saw the news of the explosion at unit 3," said Mikiko Amano, 55, who was at her home a few kilometres from the plant.

"It only raised my distrust in TEPCO. The company has been saying such a thing would not happen."

Chief government spokesman Yukio Edano says 500 people still within 20 kilometres of the plant have since been evacuated, following the 210,000 people who had already been moved from the area.

He says the blast was caused by a build-up of hydrogen in the building around the reactor.

Operators had earlier halted injection of sea water into the reactor, resulting in a rise in radiation levels and pressure.

Mr Edano says it is likely a partial meltdown has occurred at the No. 1 reactor, which experienced an explosion on Saturday.

A meltdown occurs when a reactor core overheats and causes damage to the facility, potentially unleashing radiation into the environment.

Prime minister Naoto Kan described the situation at the stricken nuclear plant as "alarming" earlier today.

A Japanese official said before today's blast that 22 people had been confirmed to have suffered radiation contamination and up to 190 may have been exposed.

Workers in protective clothing used hand-held scanners to check people arriving at evacuation centres.

Meanwhile, authorities announced they have safely cooled down two nuclear reactors at the Fukushima Daini complex, near the Fukushima Daiichi plant where the three problematic reactors are situated.

US warships and planes helping with relief efforts moved away from the coast temporarily because of low-level radiation. The US Seventh Fleet described the move as a precautionary measure.

Japan relies on nuclear energy for about a third of its power needs.

- AFP/Reuters