Japanese military helicopters dumped water on the troubled Fukushima Dai-ichi nuclear plant's Unit 3 reactor to stop radioactive waste from spewing into the air, while the U.S. warned the situation could be worse than thought.

A CH-47 Chinook helicopter dropped at least four loads of water on the reactor. Crews operated for no more than 40 minutes at a time, to limit their exposure to radiation.

Officials had initially ruled out using helicopters, saying the radiation would be too dangerous.

Earlier, a U.S. official said all of the water is gone from one of the spent fuel rod pools at the plant, meaning there is nothing to stop the fuel rods from getting hotter and eventually melting down, but Japan denied the claim.

"There is no water in the spent fuel pool and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures," chief of the U.S. Nuclear Regulatory Commission Gregory Jaczko said at a House Energy and Commerce subcommittee hearing Wednesday.

The outer shell of the rods could also explode with enough force to propel radioactive fuel over a wide area, if Jaczko is correct.

He said the problem is at the complex's Unit 4 reactor.

Jaczko did not say how the information was obtained but the organization and the U.S. Department of Energy have experts on the site.

The U.S. is also calling on Americans in Japan to stay at least 80 kilometres away from the plant. Japan's official evacuation zone is only about 20 km.

Japanese nuclear officials and Tokyo Electric Power Co., which operates the facility, have denied water is gone from the pool.

If the water at the fuel pool is dry, it severely limits what workers can do, because radiation levels will be so high. When operating correctly, the water not only cools the fuel rods, but protects workers from gamma radiation.

However, Tokyo Electric Power Co. said they were close to completing a new power line that could power cooling systems to end the crisis at the nuclear plant.

The new line could get electric-powered pumps back online, creating a steady water supply to the six troubled reactors and spent fuel storage ponds, keeping them cool.

Government officials said Wednesday that they were going to use police water cannons –- the type normally used on rioters – to spray water on the fuel storage pond at the Unit 4 reactor.

There are more problems are the beleaguered plant, the International Atomic Energy Agency in Vienna said Wednesday. The agency said temperatures at the Unit 4, 5 and 6 reactors have been rising.

Earlier Wednesday, emergency crews working at the plant were ordered to temporarily stop efforts to cool the facility's overheating reactors amid a surge in radiation levels.

At a news conference, Japan's top government spokesperson Yukio Edano said the containment vessel of one of the reactors at the plant may have been damaged, possibly sending radioactive steam into the atmosphere.

"A part of the containment vessel is broken and it seems like the vapour is coming out from there. (It) appears to be that vapour is coming out from the broken part," Edano said, explaining that the ongoing effort to spray sea water onto the reactors was disrupted by the approximately hour-long withdrawal.

Radiation levels spiked to 1,000 millisieverts per hour before coming down to the 600-800 range later in the day Wednesday.

An official with the plant's operator Tokyo Electric Power later said workers were preparing to return to their perilous work. Since 750 workers were evacuated from the plant on Tuesday, a core team of 50 workers had been rotating in and out of the facility to minimize their radiation exposure.

That number was boosted to 180 on Wednesday, the same day Japan's Ministry of Health Labour and Welfare announced it was raising the allowable radiation exposure limit for the country's nuclear workers. Describing the change as "unavoidable due to the circumstances," the ministry increased the limit from 100 millisieverts to 250.