Employees of the Tokyo Electric Power Company (TEPCO), operator of the Kashiwazaki-Kariwa nuclear plant in Niigata -- where the 6.4-magnitude quake struck -- faxed a message to local authorities seeking to allay any fears of damage.

But TEPCO workers accidentally ticked the wrong box on the form, mistakenly indicating there was an abnormality at the plant rather than there was no problem.

One official filled out the form, and it was checked by a colleague before being sent.

Many Japanese government departments and companies still rely on fax machines for communication.

TEPCO's Tokyo headquarters noticed the mistake, and a correction was published 17 minutes after the original release, the firm's Tokyo-based spokesman told AFP.

Kashiwazaki city mayor Masahiro Sakurai saw the incorrectly filled-out form and immediately directed staff to check what was happening.

The mayor hit out at TEPCO, which also operated the crippled Fukushima nuclear plant -- site of the worst nuclear accident since Chernobyl -- when an earthquake and tsunami struck in 2011.

"When a real earthquake is happening, not a drill, this is a massive error," Sakurai told local reporters, according to the Mainichi Shimbun daily.

"It is extremely poor on their part to make errors in the most important and basic information at a time of crisis," he said, according to the Asahi Shimbun newspaper.

TEPCO apologised and vowed not to make the same mistake.

The late-night quake prompted a tsunami advisory, but only small ripples of 10 centimetres (three inches) were recorded.

The government said up to 26 people were injured -- two seriously, although not life-threatening.