TOKYO — Japan is to raise its assessment of the severity of its nuclear emergency to the maximum seven on an international scale, putting it on a par with Chernobyl, reports said Tuesday.

The government currently rates the incident at five, but will boost that to seven, Kyodo and NHK said, citing unnamed sources.

The Fukushima Daiichi nuclear plant was badly damaged by the March 11 earthquake and tsunami that hit Japan and has been leaking radiation since, despite efforts to cool and stabilise its reactors.

Kyodo earlier reported that preliminary figures from the country's Nuclear Safety Commission revealed the battered plant had released 10,000 terabecquerels of radioactive material per hour for several hours.

That calculation prompted Japan to consider upgrading the accident to the highest level -- something that has only been given to the 1986 Chernobyl disaster -- Kyodo said, citing unnamed government sources.

According to the International Nuclear Events Scale, level seven incidents are ones with a "major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures."

A strong offshore earthquake rocked Tokyo on Tuesday, swaying buildings in the Japanese capital and stopping subway services.

The US Geological Agency put the magnitude at 6.4, at a depth of 13.1 kilometres (8.1 miles), 77 kilometres east of Tokyo. The Japan Meteorological Agency had measured it at 6.3.

The quake hit at 8:08 am (2308 GMT Monday) off the coast of Chiba prefecture, just east of the capital. Japanese experts said there was no chance of a tsunami.

There were no immediate reports of fresh damage, including at the crippled Fukushima Daiichi nuclear power plant, which has been releasing dangerous radioactive materials since it was damaged by the March 11 tsunami, Tokyo Electric Power Co. said.

Subway services in Tokyo temporarily stopped, but resumed operations shortly afterwards.

The runways of Narita international airport in the prefecture were temporarily closed for checks but had since reopened, Kyodo News said.

The limited shinkansen bullet train services running to the northern region since the March 11 earthquake and tsunami were briefly interrupted.

The latest quake was an aftershock "in a broad sense of the word" of the 9.0-magnitude quake of March 11 that killed more than 13,000 and left over 13,500 missing, Koshun Yamaoka, professor at Nagoya University, told public broadcaster NHK.

"We have to be aware of aftershocks, particularly in the first and second months after the original quake," he said.

Japan has experienced more than 400 aftershocks stronger than magnitude 5.0 since March 11.