Tokyo: A powerful aftershock that rocked an area of Japan still reeling from last month's earthquake and tsunami disaster killed four people and injured around 100, reports and emergency services said Friday.

The Fire and Disaster Management Agency said a 63-year-old woman in Yamagata prefecture died after her respirator failed when the power was knocked out by the 7.1 magnitude tremor.

"Her respiratory equipment was found turned off. We believe the machine was turned off due to the (power) outage," a spokesman for the agency said.

Broadcaster NHK said two men aged 79 and 85 were killed in Myagi prefecture - the area worst hit by the 9.0 magnitude quake of March 11 and the tsunami it spawned.

The Jiji news agency said an 83-year-old woman in the same prefecture was taken to hospital immediately after the earthquake and later confirmed dead.

The disaster management agency said at least 93 people were confirmed injured as of 9:30am, while Jiji Press said about 140 were injured.

The number of households still without power was 2.62 million as of noon, said Tohoku-Electric Power, the local electricity supplier.