Japanese authorities will release slightly radioactive vapor to ease pressure at a disabled nuclear power plant reactor near Tokyo .

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, the Associated Press reports.

The agency says the radioactive element in the vapor would not affect the environment or human health.

Officials have declared Japan's first ever nuclear emergency and ordered the evacuation of 3,000 nearby residents as a precaution.

The 40-year-old plant in Onahama, about 170 miles northeast of Tokyo, is not leaking radiation.

''We have a situation where one of the reactors (of the plant) cannot be cooled down,'' top government spokesman Yukio Edano said, according to Kyodo News.

The U.S. government has sent over coolant for the nuclear plant aboard U.S. Air Force planes, Secretary of State Hillary Rodham Clinton said today, according to CNN.