Japanese authorities declared a state of emergency today for five nuclear reactors at two quake-stricken power plants as military and utility officials scrambled to tame rising pressure and radioactivity levels inside the units and stabilize the facilities used to cool the plants’ hot reactor cores.

Radiation surged to around 1,000 times the normal level in the control room of one reactor, Japan’s Nuclear and Industrial Safety Agency said. Meanwhile, Tokyo Electric Power Co. said today that the temperatures at two other reactors at a different power station were rising and that it had lost control over pressure in the reactors.

Though no significant release of radioactivity had taken place, the earthquake, which forced the automatic shutdown of 11 of the country’s 55 nuclear power plants, is certain to rattle confidence in nuclear power in Japan, where people have long been sensitized to the dangers of radioactivity releases, and in the United States, where foes of nuclear power were already pointing to the Japan crisis as a warning sign.

The closure of the plants, representing nearly 20 percent of the country’s capacity, also deals an economic blow to Japan, which relies on nuclear power for one-third of its electricity.

“It’s a very serious situation for the reactors and might ultimately render those reactors unusable,” said Howard Shaffer, a former Navy submarine engineer and a member of the public information committee of the American Nuclear Society.

Japanese authorities initially evacuated about 3,000 residents living within a 1.9-mile radius of the Fukushima Daiichi plant on the east coast about 200 miles north of Tokyo and south of the heavily damaged town of Sendai. Later, they widened that evacuation to a six-mile radius. People within a 16.2-mile radius were told to remain indoors, according to the website of the International Atomic Energy Agency’s Incident and Emergency Centre.

Tokyo Electric Power, which owns the Fukushima Daiichi complex, said later that it was also having trouble controlling three of the four reactors it owns at its nearby Fukushima Daina plant.

NISA said no dangerous level of radioactivity had been released, but the government took the unusual step of evacuating nonetheless.

The problems at the nuclear plants came in waves, starting with two of the six Daiichi units.

The earthquake disrupted the electric power the reactors normally use to run their cooling facilities, which pump water into the reactor core to cool the spent fuel there.

The reactors switched to backup diesel generators, but the tsunami then swept in and shut down the generators used for the No. 2 reactor at Fukushima Daiichi. The unit then tapped excess steam in the core to power a turbine and switched to battery power, which would last only a few hours.

“There’s a basic cooling system that requires power, which they don’t have,” said Glenn McCullough, former chairman of the Tennessee Valley Authority, who was tracking the Japan situation.

Utility and Japanese government officials raced to get another generator to the site to prevent a possible partial meltdown similar to what took place in 1979 at Three Mile Island. By this morning, they said they had succeeded. The utility said it had restored power from the grid, but the IAEA said power was restored from “mobile electricity supplies.”

Meanwhile, Tokyo Electric said it had decided to vent steam and gas to relieve pressure that had increased in the containment building at unit No. 1. The company said on its website that the increase was “assumed to be due to leakage of reactor coolant.” It remained unclear where the leak was. The company said it did not think there was leakage of reactor coolant in the containment vessel “at this moment.”

There were also reports of elevated radiation levels inside the control room of that 40-year-old reactor unit. NISA said levels were 1,000 times the norm inside the room. The Associated Press later quoted an NISA official as saying that a measurement of radiation levels outside the plant was eight times as high as normal.

Even that level of radiation still posed little danger to residents, nuclear experts said. They also said the releases of steam and gas from containment buildings posed little danger, because a system of valves and filters would remove most, though not all, of the radioactivity in the steam.

The status of Tokyo Electric’s Daina plants remained unclear. Earlier, they had been said to have completed automatic shutdowns. But today, Tokyo Electric suggested that they were having problems similar to the ones at the other nuclear complex because of disruptions in the power supply needed to run cooling facilities.

“The very thermally hot reactor cores at the plant must be continuously cooled for 24 to 48 hours,” said Kevin Kamps, a specialist in nuclear waste at Beyond Nuclear, a group devoted to highlighting the perils of nuclear power. “Without any electricity, the pumps won’t be able to pump water through the hot reactor cores to cool them.”

President Obama said at a news conference that he had told Energy Secretary Steven Chu to offer help to Japan.

In a statement that confused nuclear experts, Secretary of State Hillary Rodham Clinton said Friday morning that U.S. Air Force planes in Japan had delivered “coolant” to a nuclear power plant affected by the quake. Nuclear reactors do not require special coolants, only large amounts of pumped water.

“They have very high engineering standards, but one of their plants came under a lot of stress with the earthquake and didn’t have enough coolant,” she said, “and so Air Force planes were able to deliver that.”

An Air Force spokesman at the Pentagon, however, said he was unaware of any deliveries being made by Air Force planes related to the reactor issues.

“To our knowledge, we have delivered nothing in support of the nuclear power plant,” Lt. Col. John Haynes said. “Obviously, we stand by to assist with anything they might need.” He said the Air Force had received no formal request for help.

In addition to the efforts to get Tokyo Electric’s nuclear reactors under control, Japan’s NISA said Friday that a fire had broken out at the Onagawa nuclear power plant but was later extinguished. The three reactors at the Onagawa site remained closed.

The key buildings in the Onagawa plant are about 50 feet above sea level, according to the website of Tohoku Electric Power, owner of the plant. The company said that was about twice the height of the previous highest tsunami.

The IAEA said it is seeking details on Japan’s nuclear power plants and research reactors, including information on off-site and on-site electrical power supplies, cooling systems and the condition of the reactor buildings. Nuclear fuel requires continued cooling even after a plant is shut down, the IAEA noted.

“This is the most challenging seismic event on record, so it is a severe test,” McCullough said.