Tokyo (CNN) -- Japanese authorities said radiation levels had dropped at the earthquake-damaged Fukushima Daiichi nuclear plant Tuesday
"The level has come down to the level to cause no harm to human health, according to the report I have received," Chief Cabinet Secretary Yukio Edano told reporters, describing reading taken at the plant's front gate.
Edano said readings at the gate at 3:30 p.m. (2:30 a.m. ET) were 596.4 microsieverts per hour -- compared to a high reading of 11,930 microsieverts per hour at 9 a.m (8 p.m. ET Monday).
"We have to monitor the situation closely, but the high concentration of radioactive material is not emitting constantly from the No. 4 reactor right now," he said.
But it was unclear whether an explosion and fire earlier had allowed dangerous radioactive material to escape.
Earlier Tuesday, for the first time since Friday's quake crippled cooling systems at three of the plant's reactors, Edano said radiation levels at the plant had increased to "levels that can impact human health."
The plant's owners evacuated all but about 50 workers from the facility. Edano said anyone within 30 kilometers (18.6 miles) of the plant should remain indoors.And the government imposed a no-fly zone over the 30-kilometer radius "because of detected radiation after explosions" there, the country's transportation ministry said.
"Radiation has come out from these reactors and the reading of the levels seems very high. There is still a very high risk of further radioactive material coming out," Prime Minister Naoto Kan said, asking people to remain calm.
The officials briefed reporters several hours after an explosion at the No. 2 reactor -- the third blast at the plant in four days. As they spoke, firefighters were battling a blaze at the No. 4 reactor.
Japan's economic ministry later reported that the fire had been extinguished.
Japanese officials told the International Atomic Energy Agency that radioactivity was "being released directly into the atmosphere" during the fire, which occurred at the No. 4 reactor's spent fuel storage pond, according to a statement from the UN watchdog organization.
"The Japanese authorities are saying that there is a possibility that the fire was caused by a hydrogen explosion," the statement said.
But what caused the fire -- and the extent of damage at the troubled plant -- remained unclear.
The announcement from officials Tuesday "points to something different, something more serious" after the explosion at the No. 2 reactor, CNN analyst James Walsh said. "But we don't have the definitive evidence yet."
Edano said earlier that he could not rule out the possibility of a meltdown at all three troubled reactors at the plant.
If fuel rods inside the reactors are melting, Walsh said a key detail is whether the melted material stays inside the reactor.
"The Japanese plants and all modern plants have a containment vessel. Essentially the reactor is inside of a vault. And that vault is made of thick concrete and steel," Walsh said. "The million-dollar question is whether that melting will be contained... We'll know within 24 hours. That's the key thing people should be paying attention to."
The explosion took place shortly after 6 a.m. Tuesday (5 p.m. Monday ET), Tokyo Electric Power Company said. Readings indicate some damage to the No. 2 reactor's suppression pool, a donut-shaped reservoir at the base of the reactor's containment vessel.
Afterward, Edano said radiation levels at the plant were between 100 and 400 millisieverts, or as much as 160 times higher than the average dose of radiation a typical person receives from natural sources in a year.
Workers have been injecting seawater in a last-ditch effort to cool down fuel rods and prevent a full meltdown in that reactor and two others at the plant -- No. 1 and No. 3.
The first explosion occurred at the plant on Saturday, when a hydrogen buildup caused a blast that blew the roof off the building housing the plants No. 1 reactor. Authorities said a similar explosion hit the No. 3 reactor Monday -- crippling the cooling system at the No. 2 reactor.
If the effort to cool the nuclear fuel inside the reactor fails completely -- a scenario that experts who have spoken to CNN say is unlikely -- the resulting release of radiation could cause enormous damage to the plant, and possibly release radiation into the atmosphere or water. That could lead to widespread cancer and other health problems, experts say.
Concerns about the risk of radiation release spread as the situation at the plant appeared to worsen Tuesday.
U.S. Navy personnel began limiting outdoor activities and securing external ventilation systems after instruments aboard an aircraft carrier docked in Japan detected low levels of radioactivity from the Fukushima Daiichi nuclear plant, the Navy said.
The USS George Washington was docked for maintenance in Yokosuka, about 175 miles (280 kilometers) from the plant, when instruments detected the radiation at 7 a.m. Tuesday (6 p.m. ET Monday), the Navy said in a statement.
"These measures are strictly precautionary in nature. We do not expect that any United States federal radiation exposure limits will be exceeded even if no precautionary measures are taken," the Navy said.
There are six reactors at Fukushima Daiichi, located in northeastern Japan about 40 miles (65 kilometers) south of Sendai, one of the areas worst hit by Friday's earthquake and the resulting tsunami.
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