© AFPA satellite photo of the Fukushima Daiichi plant showed the damage done in earlier explosions
A quake-stricken nuclear plant in Japan has been hit by a third explosion in four days, amid fears of a meltdown.

The blast occurred at reactor 2 at the Fukushima Daiichi plant, which engineers had been trying to stabilise after two other reactors exploded.

One minister has said it is "highly likely" that the rods might melt. Radiation levels near the plant have risen.

The crisis was sparked by a 9.0-magnitude quake and tsunami on Friday.

Thousands of people are believed to have died, and millions are spending a fourth night without water, food, electricity or gas. More than 500,000 people have been left homeless.

Staff evacuation

There is no word on the cause of Tuesday's explosion, but initial indications suggest it is not on the same scale as the previous ones.

Staff working at reactor 2 have been evacuated.

On Monday, a hydrogen blast at the Fukushima Daiichi's reactor 3 injured 11 people and destroyed the building surrounding it. That explosion was felt 40km (25 miles) away and sent a huge column of smoke into the air.

It followed a blast at reactor 1 on Saturday.

All explosions have been preceded by cooling system breakdowns - although the Tokyo Electric Power Company (Tepco) said the blasts had not penetrated the thick containment walls shielding the reactor cores.

Engineers battled into the early hours of Tuesday to try to keep water levels up in order to cool the fuel rods at reactor 2, but on two occasions the rods have been fully exposed.

Long exposure can damage the rods and raise the risk of a meltdown.

Four of the five pumps used to administer cooling sea water were believed to have been damaged by the blast at reactor 3.

Higher radiation levels were recorded on Tuesday south of Fukushima, Kyodo news agency reported.

Nearly 185,000 people have been evacuated from a 20km (12 mile) exclusion zone around the plant.