A partial meltdown was likely under way at a second nuclear reactor, a top Japanese official said Sunday, as authorities frantically tried to prevent a similar threat from a nearby unit following a catastrophic earthquake and tsunami.

Some 170,000 people have been ordered to evacuate the area covering a radius of 20 kilometres around the plant in Fukushima.

A meltdown refers to a very serious collapse of a power plant's systems and its ability to manage temperatures. A complete meltdown would release uranium and dangerous byproducts into the environment that can pose serious health risks.

Japan dealt with the nuclear threat as it struggled to determine the scope of Friday's twin disasters when the earthquake — which the Japanese Meteorological Agency has now upgraded to magnitude 9.0 from 8.8, the most powerful in its recorded history — was followed by a tsunami that ravaged its northeastern coast with breathtaking speed and power.

The U.S. Geological Survey has measured the quake at magnitude 8.9, and that number remained unchanged Sunday.

The official count of the dead was 763, but the government said the figure could far exceed 1,000. Thousands of others are unaccounted for.

In this video image taken from NTV Japan, smoke rises from the Fukushima Daiichi nuclear power plant's Unit 1 in Okumamachi, Japan. NTV Japan/Associated Press

The quake and tsunami damaged three reactors at the Fukushima Daiichi nuclear plant, which lost the cooling functions necessary to keep the fuel rods functioning properly. At first, the Unit 1 reactor was in trouble with an explosion destroying the walls of the room housing it. Later, Unit 3 also began to experience problems.

Chief Cabinet Secretary Yukio Edano said operators released slightly radioactive air from Unit 3 Sunday, while injecting water into it as an effort to reduce pressure and temperature to save the reactor from a possible meltdown.

Still, a partial meltdown in the unit is "highly possible," he told reporters.

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

Edano said radiation levels briefly rose above legal limits, but they had since declined significantly. Also, fuel rods were exposed briefly, he said, indicating that coolant water didn't cover the rods for some time. That would contribute further to raising the temperature in the reactor vessel.

Tokyo Power Electric Co., which runs the Fukushima Daiichi plant, said four workers were being treated in hospital for minor injuries after the explosion.

Edano said radiation around the plant did not increase after the explosion and actually decreased. He added that pressure in the reactor was also down.

Iodine given to residents for protection

According to Reuters, Japanese authorities have told the United Nations Atomic Energy Commission the government is preparing to hand out iodine to residents to help protect them from radiation exposure.

An official in protective gear checks for signs of radiation Saturday on a child who was near the Fukushima Daini nuclear plant in Koriyama, one of two nuclear plants in the country with evacuation areas around them. Kim Kyung-Hoon/Reuters

"People in the danger zones have been told to cover their mouths and noses with wet cloths," freelance reporter Craig Dale, told CBC News on Saturday.

He added there were also instructions for residents to be aware of the possibility of "internal exposure" and to avoid eating fruits and vegetables until they get the all-clear from authorities.

A jet stream or air current could well carry radioactive fallout as far as British Columbia, the CBC's Belle Puri reported.

The worst-case scenario would be a Chernobyl-scale catastrophe, with explosions destroying the reactors and sending a deadly plume of radioactivity into the atmosphere. The Chernobyl accident 25 years ago in Ukraine spewed radiation over much of Europe after a nuclear reactor exploded and caught fire.

But such a scenario would be highly unlikely in Japan, said Jim Walsh of the Massachusetts Institute of Technology, who is an expert on nuclear threats.

If the containment vessel around the reactor has not been touched, it should act as a layer of defence to prevent radiation from escaping, he told CBC's Mark Kelly.

The main issue is how to cool down the reactor, which has already been shut down.

Walsh said the concern was that with an increase in temperature and pressure, the fuel for the reactor could melt.

"And if it melts and somehow escapes the reactor vessel and touches the air, it will explode," he said.