TOKYO - Japanese authorities said Sunday that efforts to restart the cooling system at one of the reactors damaged by Friday's earthquake had failed, even as officials struggled to bring several other damaged reactors under control.

Workers at the Fukushima Daiichi nuclear plant have not found a way to stabilize overheated reactors and feared the possibility of partial nuclear meltdown, which could potentially cause a further release of radioactive material, Japan's top government spokesman said Sunday. Engineers were having trouble, in particular, with two units at the nuclear facility - one of which lost its outer containment wall Saturday in an explosion.

Meanwhile, officials declared a state of emergency at a nuclear power plant in Onagawa, where excessive radiation levels were reported.

Chief Cabinet Secretary Yukio Edano said a similar explosion could soon occur at Fukushima Daiichi's unit 3, the result of hydrogen levels that are increasing within the unit's reactor vessel amid last-ditch efforts to keep fuel rods submerged in water. Already, trace amounts of radioactive material have leaked from the No. 3 reactor, Edano said.

"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said.

But Edano also insisted that an explosion would have no impact on human health. Based on initial findings from the government and from Japan's nuclear agency, the Saturday explosion in unit 1 did not damage the reactor vessel, and the government said that the unit 3 reactor vessel would also withstand an explosion. The reactor vessels of No. 3 and No. 1 are being flooded with seawater and boron in an emergency attempt to keep the units cool after the plant lost its main power supply and a backup system failed.

Though the third unit is being filled with water, its gauge inside does not register the rising levels, Edano said. He did not have an explanation.

"If the cooling system is not maintained, there is a good chance the core could start melting down," said Masahi Gota, a former Toshiba engineer who was involved in the design of the containment vessel for these nuclear reactors.

Richard Lester, co-chair of the Department of Nuclear Science and Engineering at MIT, said: "The most important task that the operators have - and have had for last 36 hours - is to keep the fuel in the reactor covered, submerged in water. If they succeed in doing that, keeping the fuel rods covered in water, the likelihood of significant damage to the fuel is low. If they cannot keep the fuel covered with water, then you have the possibility of melting."

Some 170,000 people have been evacuated around a 12-mile radius of the plant. They join more than 450,000 other evacuees from other quake- and tsunami-affected regions. A spokesman for Japan's nuclear agency said as many as 160 people may have been exposed to radiation and were being tested at a hospital to determine if levels were dangerous.

"Only the gravest danger would justify an evacuation at such a moment," said Peter Bradford, a former commissioner at the U.S. Nuclear Regulatory Commission.

Edano said officials were acting on the assumption that a meltdown could be underway at Fukushima Daiichi's unit 3, and that it was "highly possible" a meltdown was underway at its unit 1 reactor, where an explosion destroyed a building a day earlier.