Japan's nuclear safety agency raised the severity rating of the crisis at its nuclear plant to the highest level today, on a par with the 1986 Chernobyl disaster.

An official with the Nuclear Safety Commission of Japan, speaking on national television, said the rating was raised from five to seven.

The official said the amount of radiation leaking from the Fukushima Dai-ichi nuclear plant was around 10% of that in the Chernobyl accident in the former Soviet Union.

Level 7 signifies a "major accident" with "wider consequences" than the previous level, according to the standards scale.

"We have upgraded the severity level to 7 as the impact of radiation leaks has been widespread from the air, vegetables, tap water and the ocean," said Minoru Oogoda of Japan's Nuclear and Industrial Safety Agency (Nisa).

Nisa officials said one of the factors behind the decision was that the total amount of radioactive particles released into the atmosphere since the incident had reached levels that applied to a Level 7 incident.

The action lifts the rating to the highest on an international scale designed by a group of experts in 1989 and is overseen by the International Atomic Energy Agency.

In Chernobyl, in the Ukraine, a reactor exploded on April 26 1986, spewing a cloud of radiation over much of the Northern Hemisphere. A zone about 19 miles around the plant was declared uninhabitable, although some plant workers still live there for short periods and a few hundred other people have returned despite government encouragement to stay away.

Meanwhile, setbacks continued at Japan's tsunami-stricken nuclear power complex, with workers discovering a small fire near a reactor building at 8.38pm BST yesterday. The fire was extinguished quickly, the plant's operator said.

Tokyo Electric Power Co (Tepco) which operates the disabled Fukushima Dai-ichi nuclear power plant, said the fire at a box that contained batteries in a building near the No 4 reactor was discovered at about 6.38am Tuesday, local time, and was put out seven minutes later.

It was not clear whether the fire was related to a magnitude-6.3 earthquake that shook the Tokyo area yesterday. The cause of the fire is being investigated.

"The fire was extinguished immediately. It has no impact on Unit 4's cooling operations for the spent fuel rods," said Tepco spokesman Naoki Tsunoda.

The plant was damaged in a massive tsunami on March 11 that knocked out cooling systems and back-up diesel generators, leading to explosions at three reactors and a fire at a fourth that was undergoing regular maintenance and was empty of fuel.

The magnitude-9.0 earthquake that caused the tsunami immediately stopped the three reactors, but overheated cores and a lack of cooling functions led to further damage.

Engineers have been able to pump water into the damaged reactors to cool them down, but leaks have resulted in the pooling of tons of contaminated, radioactive water that has prevented workers from conducting further repairs.

Aftershocks yesterday briefly cut power to back-up pumps, halting the injection of cooling water for about 50 minutes before power was restored.

The revision was based on cross-checking and assessments of data on leaks of radioactive iodine-131 and caesium-137, said Nisa spokesman Hidehiko Nishiyama.

"We have refrained from making announcements until we have reliable data. The announcement is being made now because it became possible to look at and check the accumulated data assessed in two different ways," he said, referring to measurements from Nisa and the Nuclear Security Council.

Mr Nishiyama noted that unlike in Chernobyl there had been no explosions of reactor cores at the Fukushima Dai-ichi plant, although there were hydrogen explosions.

"In that sense, this situation is totally different from Chernobyl," he said.

Tepco was still estimating the total amount of radioactive material that might be released by the accident, said company spokesman Junichi Matsumoto.

He acknowledged the amount of radioactivity released might even exceed the amount emitted by Chernobyl.

The company, under fire for its handling of the accident and its disaster preparedness before the March 11 earthquake and tsunami, issued yet another apology.

"We humbly accept this. We deeply apologise for causing tremendous trouble to those who live near the nuclear complex and people in the prefecture," Tepco spokesman Naoki Tsunoda said.

A month after the disaster, more than 145,000 people are still living in shelters, and the government yesterday added five communities to a list of places people should leave to avoid long-term radiation exposure.

A 12-mile radius has already been cleared around the plant.

The disaster is believed to have killed more than 25,000 people, but many of those bodies were swept out to sea and more than half of those feared dead are still listed as missing.

Aftershocks have taken more lives.

In Iwaki, a city close to the epicentre of a magnitude-7.0 temblor yesterday, a landslide brought down three houses, trapping up to seven people.

Four were rescued alive, but one of those - a 16-year-old girl - died at the hospital, a police official said.

Around 210,000 people have no running water and following yesterday's aftershocks, more than 240,000 people are without electricity.

In all, nearly 190,000 people have fled their homes, the vast majority of whom are living in shelters, according to the national disaster agency. About 85,000 are from the cleared zone around the nuclear plant. Their homes may be intact, but it is not known when they'll be able to return to them.