Emergency crews have once more been evacuated from the Fukushima nuclear plant, after black smoke began belching out of one of the stricken reactors. Engineers have been working to cool the reactors and spent fuel rods to avoid a catastrophic release of radiation.

Contamination levels were reported to be unusually high before the smoke was spotted. They have since fallen back but remain higher than in recent days.

Power cables have been connected to all reactors.

But engineers were forced to halt tests of the system at reactor 2 after radiation levels spiked.

There is also concern about rising temperatures at reactor 1.

Experts warn powering up all the reactors could take weeks or even months.