The admission came as the threat level of the nuclear crisis was increased from 4 to 5 on a 7-level international scale.
The threat Level 5 at Fukushima is the same level the Three Mile Island plant reached back in 1979. The Chernobyl disaster which killed 31 people back in 1986 was rated as Level 7.
The Japanese government admitted it has been overwhelmed by the scale of the twin disasters that hit the country, which has slowed its response to the nuclear crisis. It's also now welcoming U.S. help to stabilize the plant.
Even though the threat level increased, there has been some progress at the crippled Fukushima Dai-ichi nuclear plant.
Military fire trucks are being used to try and get water on the plant, which could help cool the nuclear fuel. Workers started doing that Thursday along with helicopter drops of water.
Crews at the plant are getting help from firefighters who were brought in to help with the cooling efforts. The 39 men all volunteered for what is considered extremely dangerous work.
"I certainly think that they're fully aware of the threats to their own health. Radiation has no smell, so they cannot tell if whether they are being exposed to lethal doses," said nuclear analyst Michael Dobbs.
Some power has been restored to the reactor, allowing workers to use the facility's own cooling system to pump water into two of the reactors.
Damage to the facility is extensive after several explosions. Japanese officials raised the severity level of the accident because the scope of the disaster has spread beyond Japan.
Officials said they realized that at least 3 percent of the fuel in three of the reactors at the power plant had been severely damaged, suggesting those reactor cores have partially melted down and thrown radioactivity into the environment.
The head of the International Atomic Energy Agency has arrived in Japan to assess the situation.
"We see it as an extremely serious accident," said Yukiya Amano, IAEA Director General. "The international community is concerned about this issue. So it is important to cooperate with the international community in dealing with it."
A level 4 incident only impacts the local area, whereas a level 5 incident affects a bigger area. The 1979 Three Mile Island accident was a level five and the meltdown at Chernobyl, which killed 31 people, was rated a level seven.
Obama Administration orders review of nuclear facilities in U.S.
In the wake of the nuclear disaster in Japan, the Obama Administration is ordering a complete review of nuclear facilities in the U.S.
There are two plants in California, one at San Onofre near San Clemente and one in Diablo Canyon in Avila Beach.
U.S. officials said they believe reactors in the country are safe, but they feel a review is the prudent thing to do.
Some of the plants in the U.S. use the same reactor design as the Fukushima plant.
Japan marks one week since disasters struck
People in Japan paused for a moment of silence Friday to mark one full week since the magnitude-9.0 earthquake and deadly tsunami hit.
Some people returned to their homes to see what they can salvage or clean up.
More than 6,900 people are confirmed dead, with around 10,700 still missing.
More than 450,000 people are homeless and some are staying in shelters. In cities that do have electricity, rolling blackouts are expected to last for months.
Japan's prime minister had some sobering comments, saying people should brace to rebuild their country from scratch. He reminded the public that it's not the first time Japan has been in a dire situation.
The Associated Press contributed to this report.