Japan's military CH-47 Chinook helicopters began spraying tonnes of water on reactors 3 and 4 at Fukushima, 220km (140 miles) from Tokyo, at 0948 local time (0048 GMT), officials said.
Two helicopters, flying at less than 300 feet, dumped four loads of water on the reactor, although the footage suggested a significant quantity was missing the target.
Initially police crews had tried to spray the reactor but were forced to withdraw because they would have been exposed to high radiation levels. The military vehicles, unlike those of the police, are built to allow personnel to remain inside, NHK reported.
The roofs of the No 3 and No 4 reactors were blown away by hydrogen explosions earlier this week, depriving them of a last line of defence against potentially dangerous radiation leaks.
In the worst-case scenario, overheating fuel rods could heat up to the point where they begin to melt and release high levels of radioactivity.
Japan's top spokesman, Yukio Edano, said there was no need to widen the exclusion zone, but signs are emerging that other countries are taking a more cautious approach.
The worsening situation prompted the US to ask citizens living within an 80-km radius to evacuate.
''We are recommending, as a precaution, that American citizens who live within 50 miles (80km) of the Fukushima nuclear power plant evacuate the area or to take shelter indoors if safe evacuation is not practical,'' the US embassy said in a statement.The British embassy has since issued similar advice, and asked citizens living in Tokyo and northern Japan to consider leaving.