The focus of Japan''s nuclear crisis switched to reactor No. 4 at the troubled Fukushima Daiichi power station on Tuesday after an explosion and fire blasted holes in the unit''s outer building.

Japanese media said a pool used to store spent nuclear fuel at the reactor, which shut when the earthquake and tsunami struck last week, might be boiling. Radiation levels near Tokyo briefly reached more than 10 times above normal before returning closer to normal levels by evening.

Reactor operator Tokyo Electric Power Co (TEPCO) said the explosion damaged the roof in the No. 4 building, where the fuel was stored inside. The fire, it said, was put out with help from the US military.

The situation is potentially more dangerous than the explosions that rocked the No. 1, 2 and 3 reactors at the plant in Fukushima, 240 km (150 miles) north of Tokyo, since the spent fuel pool does not lie within the double layers of containment and thick steel walls that shroud a reactor''s core.

"They''ve got to get the fires out, keep the fires out and keep the water from boiling," said Murray Jennex, a professor at San Diego State University in California.

Signs to watch for would be rising radiation levels and what sort of particles are being emitted, with plutonium and uranium in particular signalling problems.

Authorities said they may use helicopters to pour water on reactor No. 4 within two or three days but did not say why they would have to wait to do this.

Experts had expressed concern about the No. 2 reactor, where Jennex said there had been a real possibility of a leak in the reactor container, which houses the nuclear fuel rods.

Concerns centred on damage to a part of the reactor core known as the suppression pool, which helps to cool and trap the majority of cesium, iodine and strontium in its water.

The nature of the damage and its impact on the integrity of the containment structure, a thick steel vessel that surrounds the core, are unclear.

Japan''s nuclear crisis now appears worse than the Three Mile Island nuclear accident in the US state of Pennsylvania in 1979 but still nowhere near as bad as the Chernobyl disaster in Ukraine in 1986.

Neighbouring countries could be the first to sound the wider alarm about a major radioactive leak, as happened with Chernobyl. In Japan''s case, that would most likely be South Korea, China or Russia.