TOKYO (AP) -- Japan's massive earthquake caused a power outage that disabled a nuclear reactor's cooling system, triggering evacuation orders for about 3,000 residents as the government declared its first-ever state of emergency at a nuclear plant.

Japan's nuclear safety agency said pressure inside one of six boiling water reactors at the Fukushima Daiichi plant had risen to 1.5 times the level considered normal. To reduce the pressure, slightly radioactive vapor may be released. The agency said the radioactive element in the vapor would not affect the environment or human health.

After the quake triggered a power outage, a backup generator also failed and the cooling system was unable to supply water to cool the 460-megawatt No. 1 reactor, though at least one backup cooling system is being used. The reactor core remains hot even after a shutdown.

The agency said plant workers are scrambling to restore cooling water supply at the plant but there is no prospect for immediate success.

Chief Cabinet Secretary Yukio Edano said the 40-year-old plant was not leaking radiation. The plant is in Onahama city, about 170 miles (270 kilometers) northeast of Tokyo.

If the outage in the cooling system persists, eventually radiation could leak out into the environment, and, in the worst case, could cause a reactor meltdown, a nuclear safety agency official said on condition of anonymity, citing sensitivity of the issue.

Another official at the nuclear safety agency, Yuji Kakizaki, said that plant workers were cooling the reactor with a secondary cooling system, which is not as effective as the regular cooling method.

Kakizaki said officials have confirmed that the emergency cooling system - the last-ditch cooling measure to prevent the reactor from the meltdown - is intact and could kick in if needed.

"That's as a last resort, and we have not reached that stage yet," Kakizaki added.

Japan's nuclear safety agency said the evacuation, ordered by the local government of Fukushima, affects at least 2,800 people. Edano said residents were told to stay at least two miles (three kilometers) from the plant and to stay inside buildings.

He said both the state of emergency and evacuation order are meant to be a precaution.

"We launched the measure so we can be fully prepared for the worst scenario," he said. "We are using all our might to deal with the situation."

Defense Ministry official Ippo Maeyama said the ministry has dispatched dozens of troops trained for chemical disasters to the Fukushima plant in case of a radiation leak, along with four vehicles designed for use in atomic, biological and chemical warfare.

High-pressure pumps can temporarily cool a reactor in this state with battery power, even when electricity is down, according to Arnold Gundersen, a nuclear engineer who used to work in the U.S. nuclear industry. Batteries would go dead within hours but could be replaced.

It was not immediately clear how many of the site's six reactors were affected by the cooling problem.

Speaking at the White House, Secretary of State Hillary Clinton also said U.S. Air Force planes were carrying "some really important coolant" to the site. She said "one of their plants came under a lot of stress with the earthquake and didn't have enough coolant."

Neil Sheehan, a spokesman for the U.S. Nuclear Regulatory Commission, said staff were trying to collect more information on what was happening.

At the Fukushima Daiichi site, "They are busy trying to get coolant to the core area," Sheehan said. "The big thing is trying to get power to the cooling systems."

Meanwhile, plant operator Tokyo Electric Power Co. said in a statement that closures of the plants in the quake-hit region could result in less power generation.

The plant is just south of the worst-hit Miyagi prefecture, where a fire broke out at another nuclear plant. The blaze was in a turbine building at one of the Onagawa power plants; smoke could be seen coming out of the building, which is separate from the plant's reactor, Tohoku Electric Power Co. said. It has since been extinguished.

Another reactor at Onagawa was experiencing a water leak.

The U.S. Geological Survey said the 2:46 p.m. quake was a magnitude 8.9, the biggest earthquake to hit Japan since officials began keeping records in the late 1800s.

A tsunami warning was issued for a number of Pacific, Southeast Asian and Latin American nations.

At the two-reactor Diablo Canyon plant at Avila Beach, Calif., an "unusual event" - the lowest level of alert - was declared in connection with a West Coast tsunami warning. The plant remained stable, though, and kept running, according to the NRC.

---

AP National Writer Jeff Donn reported from Boston