JOIN USThe catastrophic quake and ensuing tsunami have left 5,178 people dead. A total of 8,606 people were still unaccounted for till Thursday morning, the National Police Agency said.
The military's CH-47 Chinook helicopters dumped four loads of water on reactors three and four at Fukushima, 220 km from Tokyo, at 9.48 a.m., BBC said.
The water dumping operation by the army came after the plant operator - the Tokyo Electric Power Co - failed in its efforts, Xinhua reported.
On Wednesday, the helicopters - capable of carrying up to 7.5 tonnes of water each - were forced to abort a similar operation amid concerns over high radiation levels.
Japanese government spokesman Yukio Edano said nuclear experts were investigating how effective the operation was.
A Tokyo police unit is also using a water cannon truck to cool down a spent fuel rod pool at the No.4 reactor at the quake-hit nuclear power plant.
The nuclear plant was rocked by a hydrogen explosion Tuesday as well as a fire Wednesday, raising concerns that the fuel rods will melt and release radiation.
The head of the International Atomic Energy Agency (IAEA), Yukio Amano, is travelling to Japan to gather more information.
The US, Britain and France have advised their nationals currently living in Tokyo to consider leaving the area.
In areas badly hit by the tsunami, the chilly winter weather has added to the misery of survivors. Officials in Fukushima prefecture said evacuation centres lack basic necessities, including sufficient hot food.
About 450,000 people are staying in temporary shelters, many sleeping on the floors.