A second explosion has rocked Japan's troubled Fukushima Dai-ichi nuclear power plant, leaving three workers injured and seven other missing, Tokyo Electric Power Co. said.

A similar blast occurred at the plant's Unit 1 reactor on Saturday.

Yukio Edano, Japan's chief cabinet secretary, confirmed that a hydrogen explosion occurred at the Unit 3 reactor on Monday morning, local time. He said people within 20 kilometres of the nuclear plant were ordered to stay inside following the explosion.

He added that the reactor's inner containment vessel, which holds nuclear rods, remained intact.

Government officials had warned on Sunday that an explosion was possible at the Unit 3 reactor. When it came, it was powerful enough that Associated Press journalists felt the blast from 25 kilometres away.

More than 180,000 people have been evacuated from the area surrounding the Dai-ichi plant.

Smoke could be seen rising from the complex in the wake of the blast, which occurred at around 11 a.m. local time.

The explosion came as authorities struggled to bring the country's nuclear crisis under control, days after a powerful earthquake and tsunami rocked Japan's eastern coast.

Naoki Kumagai, an official at Japan's Nuclear and Industrial Safety Agency, said earlier Monday that radiation at the worst-hit Fukushima Dai-ichi nuclear power plant remained above acceptable levels.

Three other nuclear complexes reported damage after twin disasters struck the island nation on Friday. States of emergency were declared at six nuclear reactors where cooling systems were knocked out, three at Dai-ichi and three at the nearby Fukushima Daini plant.

But the situation at the Fukushima Dai-ichi appeared to be the most serious.

Radiation leaked into the air on Saturday when steam pressure was released to help cool the Unit 1 reactor. The release caused the earlier explosion that took down the walls around Unit 1, but not the steel envelope encasing it.

Chief Cabinet Secretary Yukio Edano had warned Sunday that a hydrogen explosion could also occur at Unit 3. Operators were trying to reduce pressure and heat inside the reactor, he said, but a partial meltdown was "highly possible."

"Because it's inside the reactor, we cannot directly check it," he said. "But we are taking measures on the assumption of the possible partial meltdown."

Radiation concerns

Officials had said the amount of radiation leaked did not pose health risks, although the assertion did little to ease public concern.

"First I was worried about the quake. Now I'm worried about radiation," Kenji Koshiba, a construction worker who lives near the plant, told The Associated Press from an evacuation centre.

Japanese authorities said they were making preparations to distribute iodine pills to residents near the two Fukushima plants. The pills can help protect against thyroid cancer, since they inhibit the thyroid's absorption of radioactive iodine from the atmosphere.

Up to 160 people who had been waiting for evacuation in the nearby town of Futabe may have been exposed to radiation, according to a spokesperson for Japan's nuclear agency. The severity of their exposure, or if it had reached dangerous levels, was not clear.

Extent of danger

Officials haven't said how high the temperature has risen inside Dai-ichi's troubled reactors. If temperatures reach 2,000 Celsius, the uranium pellets inside their fuel rods will start to dissolve, beginning a meltdown.

A complete meltdown could release uranium and other toxic nuclear byproducts into the environment.

It wasn't clear whether the fuel rods were melting inside the Unit 1 reactor.

Engineers were pumping in seawater to Unit 1 and Unit 3 to control the overheating, a move that international experts say is a measure of last-resort.

The cooling system at Fukushima Dai-ichi's Unit 2 has also failed, but is less affected in the crisis. Three other reactors at the facility are in a safe, shut-down state.

Japanese officials said they would institute rolling blackouts in Tokyo and other cities starting on Monday in order to manage a severe power shortage caused by the inoperable nuclear power plants.

Japan has 55 nuclear reactors housed at 17 complexes.

With files from The Associated Press