Workers have entered a reactor building at Japan's stricken nuclear plant for the first time since an explosion hit the facility a day after the 11 March earthquake and tsunami, officials said.

Wearing gas masks and protective suits with oxygen tanks on their backs, two workers stepped into the building housing reactor number one to gauge radiation levels.

The reactor is one of four units badly damaged at the Fukushima Daiichi plant.

'It was the first entry into the reactor building by our plant workers since the explosion,' said Satoshi Watanabe, a spokesman for operator Tokyo Electric Power Co (TEPCO).

The company plans to send at least 12 workers inside during the day to set up an air ventilation system that aims to cut levels of radioactivity in the air within the reactor building.

'We are sending workers as a small group for a maximum of ten minutes so that radiation they will be exposed to can be limited,' Mr Watanabe added.

'We hope to operate the ventilator later in the day.'

TEPCO will then begin building a new cooling system outside the reactor - with water pipes connecting it to heat exchange equipment inside - in an attempt to regulate temperatures in the reactor since it began overheating following the twin natural disasters.

TEPCO plans to complete construction on the new cooling system in late May or early June, local media said. Engineers aim to achieve stable 'cold shutdowns' toward the end of the year.

The reactor had been too dangerous for humans to enter.

The Fukushima plant, northeast of Tokyo, was engulfed by the tsunami that followed the nation's biggest earthquake, and rocked by a series of explosions and fires.

It has been releasing radioactive materials into the environment in the world's worst nuclear disaster since Chernobyl in 1986.