Image caption Aerial images show the extent of damage to the No 1 reactor building at Fukushima Daiichi plant

Workers at Japan's Fukushima nuclear plant have entered one of its reactor buildings for the first time since it was hit by a powerful earthquake on 11 March, officials say.

They have installed a ventilation system in the No 1 reactor to filter out radioactive material from the air.

The quake disabled reactor cooling systems, causing fuel rods to overheat.

Radiation levels inside reactor buildings must be lowered before new cooling systems can be installed.

The No 1 reactor was one of four damaged by explosions in the days immediately after the earthquake and tsunami. Water is being pumped in to cool the reactors.

New systems

The Tokyo Electric Power Company (Tepco) said a total of 12 engineers worked inside the reactor building in small teams in shifts of 10 minutes for about an hour.

The ventilator system and filters are reported to be running.

Tepco said it would then take about three days to vent the contaminated air, filter it, and return purified air to the building - allowing workers to remain inside the reactor for longer periods.

"After that we plan to start work on actually installing the cooling system," spokesman Naoyuki Matsumoto said.

Footage filmed by cameras mounted on robots sent into the reactor had already established that there were no leaks of potentially radioactive water inside the building.

Image caption Tepco President Masataka Shimizu has apologised to local residents evacuated from around the plant

The company faces similar problems at three other reactors at the six-reactor plant.

Tepco is also dealing with highly radioactive waste-water leaking from the No 2 reactor which it is moving to secure storage on site.

Tepco has said it expects to bring the crisis under control and achieve a cold shutdown of the plant by the end of the year, but some doubt whether this target can be achieved.

A 20km (12 mile) evacuation zone has been put in place around the plant because of concern about radiation levels - forcing some 80,000 residents to leave their homes.

On Wednesday residents living in a temporary evacuation centre shouted at Tepco President Masataka Shimizu when he visited them, accusing him of lying about safety levels at the plant.

"I just want to say that we are doing everything that we can, and that I want to apologise from the very bottom of my heart. I am deeply sorry," he responded.

A total of 14,785 people are confirmed to have been killed by the earthquake and resulting tsunami. Another 10,271 remain missing, according to the latest police figures.

Japan's recovery bill has been estimated at $300bn (£184bn) - making this already the most expensive disaster in history.