The report came as the government expanded an evacuation zone around the crippled Fukushima nuclear plant because of the high levels of accumulated radiation since a 15-metre tsunami ripped through the complex a month ago, causing massive damage to its reactors which engineers are still struggling to control.
The Kyodo report said that the high levels of radiation that have been released by the Fukushima Daiichi plant meant it could raise the severity level from 5 to the highest 7, the same as the 1986 Chernobyl accident.
Kyodo said the government's Nuclear Safety Commission had estimated that at one stage the amount of radioactive material released from the reactors in northern Japan had reached 10,000 terabequerels per hour for several hours, which would classify the incident as a major accident according to the INES scale.
The International Nuclear and Radiological Event Scale (INES), published by the International Atomic Energy Agency, ranks nuclear incidents by severity from 1 to a maximum of 7.
Kyodo did not say when the big increase in radiation had happened but quoted the commission as saying the release had since fallen to under 1 terabecquerel per hour.
The commission also released a preliminary calculation for the cumulative amount of external exposure to radiation, saying it exceeded the yearly limit of 1 millisieverts in areas extending more than 60 km to the northwest of the plant and about 40 km to the south-southwest.
"It has been obvious all along this was a 7. There are three reactors that are not being cooled (No. 1, 2 and 3) and four fuel pools too (No. 1, 2, 3, and especially 4)," said Arnie Gundersen, a veteran of the nuclear industry who worked on reactors similar to those at Daiichi and who is now chief engineer at Fairewinds Associates Inc of Burlington, Vermont.
He said that meant there were at least seven cores or pools that had been in difficulty. He noted that at Chernobyl it was only one reactor that created the problem.
Japan had previously assessed the accident at reactors operated by Tokyo Electric Power Co (TEPCO) at level 5, the same level as the Three Mile Island accident in the United States in 1979.
The tsunami was triggered by March 11 9.0 magnitude earthquake, the largest recorded in quake-prone Japan, crippling the reactors' cooling systems.
TEPCO said on Monday it had stopped the discharges of low-level radioactive water into the sea that have drawn complaints from neighbouring China and South Korea.
It has already pumped 10,400 tonnes of low-level radioactive water into the ocean to free up storage capacity for highly contaminated water from the reactors.
On Monday, shortly after Japan marked one month since the quake, a huge aftershock shook a wide swathe of eastern Japan, killing two people, and knocking out power to 220,000 homes.
It was one of more than 400 aftershocks above a 5 magnitude to have hit the area since March 11.
Because of accumulated radiation contamination, the government is encouraging people to leave certain areas beyond its 20 km (12 mile) exclusion zone around the plant. Thousands of people could be affected by the move.
TEPCO President Masataka Shimizu visited the area on Monday for the first time since the disaster. He had all but vanished from public view apart from a brief apology shortly after the crisis began and has spent some of the time since in hospital.
"I would like to deeply apologise again for causing physical and psychological hardships to people of Fukushima prefecture and near the nuclear plant," said a grim-faced Shimizu.
Dressed in a blue work jacket, he bowed his head for a moment of silence with other TEPCO officials at 2:46 p.m. (6:46 a.m. British time), exactly a calendar month after the earthquake hit.
Engineers at the plant north of Tokyo said they were no closer to restoring the plant's cooling system, which is critical to bring down the temperature of overheated fuel rods and to bringing the six reactors under control.
In a desperate move to cool the highly radioactive fuel rods, TEPCO has pumped water onto reactors, some of which have experienced partial meltdown.
But the strategy has hindered moves to restore the plant's internal cooling system as engineers have had to focus on how to store 60,000 tonnes of contaminated water.
Engineers are also pumping nitrogen into reactors to counter a build-up of hydrogen and prevent another explosion sending more radiation into the air, but they say the risk of such a dramatic event has lowered significantly since March 11.
The triple disaster is the worst to hit Japan since World War Two, leaving nearly 28,000 dead or missing and rocking the world's third-largest economy. ($1=85.475 Japanese yen)
(Additional reporting by Nathan Layne in Tokyo and Scott DiSavino in New York; Writing by Jonathan Thatcher)