A second explosion has rocked an earthquake-hit nuclear plant, as Japan struggles to avert a catastrophic reactor meltdown caused by a quake and tsunami feared to have killed more than 10,000.

A new tsunami scare triggered evacuations on the devastated north-east coast after a large wave was spotted rolling in to shore, but authorities said they had detected no sign of a tsunami or a quake that would have caused it.

A screen grab from local news footage shows the blast at the Fukushima nuclear power station. Credit:AFP/HO/NKH

Japan has been battling to control two overheating reactors at the ageing Fukushima plant after the cooling systems were knocked out by Friday's 8.9-magnitude quake and the resulting tsunami that swallowed up whole towns.

Shortly after Prime Minister Naoto Kan said the plant was still in an "alarming" state, a blast at its No.3 reactor shook the facility and sent plumes of smoke billowing into the sky.