TOKYO — Japan's post-earthquake nuclear crisis lurched further towards disaster early today after an explosion and a fire hit two further reactors, leading officials to warn of dangerously high levels of radiation leaking from the area.

TOKYO — Japan's post-earthquake nuclear crisis lurched further towards disaster early today after an explosion and a fire hit two further reactors, leading officials to warn of dangerously high levels of radiation leaking from the area.

An early morning blast at the No.2 reactor at Fukushima's No.1 power plant was followed by news of a blaze at the No.4 reactor — meaning four out of six reactors at the site 150 miles northeast of Tokyo now are in trouble.

Explosions already have hit the No.1 and No.3 reactors after Friday's monster earthquake and subsequent tsunami.

Delivering a message to the nation, Prime Minister Naoto Kan said people within a 12-mile radius of the plant should evacuate, while those a farther six miles away should keep indoors and shut windows.

"Radiation has spread from these reactors, and the level seems very high," Kan warned, with officials confirming the levels posed a threat to human health.

"There is no doubt that unlike in the past, the figures are the level at which human health can be affected," said chief government spokesman Yukio Edano.

He said radioactive substances might spread outside the 12- to 18-mile area but would dissipate the farther they spread. The wind in the region is forecast to blow inland for most of today.

Small amounts of radioactive substances later were detected in Tokyo, according to Kyodo News.

Kan said people still working at the plant were putting themselves in a very dangerous situation in a bid to stop any more explosions or leaking of radiation. "I would like to ask the nation — although this is an incident of great concern — I request that you act very calmly," he said.

Authorities said everything possible was being done to put out the fire at the No.4 reactor and played down the danger."Please keep in mind that what is burning is not nuclear fuel itself," Edano said. "We'll do our best to put out or control the fire as soon as possible."

Kyodo News later reported the fire had been extinguished.

Earlier, officials confirmed a blast had hit the No.2 reactor at 6:10am local time today. The explosion was described as "huge" by a spokesman for the Tokyo Electric Power Co., or TEPCO, and workers were evacuated from the reactor area, according to the Jiji news agency.

Japan's nuclear safety agency said TEPCO believed the seal around the reactor, which is critical for preventing a major radiation leak, had not been holed. However, Edano said there could be damage to the suppression pool, which forms the base of the container vessel that seals the fuel rods.

Higher radiation levels earlier were recorded in Ibaraki prefecture north of Tokyo after the blast, Kyodo News reported, but it quoted the safety agency as saying that the level did not pose health risks.

In the aftermath of Friday's 9.0-magnitude earthquake — upgraded Monday from 8.9 by the U.S. Geological Survey — and tsunami, the country's nuclear plants have lurched from one catastrophe to the next.

Officials have struggled to prevent meltdowns at the damaged reactors, saying fuel rods may have been critically damaged by overheating.