The government announced the tests just weeks after declaring that the nuclear plants were safe to restart.

Officials gave no timescale, sparking concern that the country could face power outages all year.

The 11 March earthquake and tsunami wrecked the Fukushima Daiichi plant, causing a review of nuclear power.

Engineers are still working to shut down the plant, where three reactors melted down and radioactive material has leaked out.

The 9.0-magnitude earthquake - the country's most powerful on record - created a massive tsunami that levelled towns along the country's north-east coast.