Self-Defense Forces helicopters dropped water on the troubled No. 3 reactor of the Fukushima nuclear power plant on Thursday morning as it is feared the reactor may have released radioactive steam due to damage to its containment vessel.

The Defense Ministry had given up on the deployment of Ground Self-Defense Force helicopters due to the high radiation level Wednesday.

Water cannon trucks have also been called up in an attempt to cool a spent fuel rod pool in a bid to contain the disaster at the troubled Fukushima No. 1 nuclear power station. The cannons are strong enough to allow emergency workers to remain a safe distance from the complex while still able to get water into the pool.

Officials said they aim to cool the possibly overheating pool of the No. 4 reactor to prevent the discharge of highly-concentrated radioactive materials. Tokyo Electric Power Co (TEPCO), the operator of the nuclear plant, will install temporary electric power supply to activate pumps to send water to reactors and pools.

The building housing the No. 4 reactor suffered what appeared to be a hydrogen explosion early Tuesday following the devastating earthquake and tsunami last week. A fire also broke out Wednesday there.

In the face of the extraordinary nuclear crisis, the focus is now on how to cool the possibly overheating pools that store spent nuclear fuel rods at the No. 4 as well as No. 3 reactors.

Unless the spent fuel rods are cooled down, they could deteriorate and emit radioactive materials.

The U.S. military, meanwhile, will operate a Global Hawk unmanned high-altitude reconnaissance aircraft, possibly on Thursday, to take images of the inside of the building that houses the No. 4 reactor, according to Japanese government sources.

Among the six reactors at the Fukushima No. 1 nuclear plant, part of the No. 2 reactor’s containment vessel, critical to holding harmful radioactive substances, suffered damage in the pressure-suppression chamber connected to the vessel following Tuesday’s apparent hydrogen explosion.

An estimated 70% of the nuclear fuel rods have been damaged at the plant’s No. 1 reactor and 33% at the No. 2 reactor, Tokyo Electric said Wednesday.

The cores of the No. 1, No. 2 and No. 3 reactors are believed to have partially melted with their cooling functions lost in the wake of Friday’s magnitude 9.0 earthquake and tsunami.

The government’s Nuclear and Industrial Safety Agency said Wednesday that the water level had dropped in the No. 5 reactor, which was not in service when the killer quake jolted northeastern Japan, posing the risk of overheating. The agency said it will closely monitor data on the reactor to prevent the problems that occurred at other reactors.

The government has set the evacuation zone covering areas within a 20 kilometer radius of the plant.

Meanwhile, U.S. Nuclear Regulatory Commission Chairman Gregory Jaczko said in Washington on Wednesday that all the water was gone from the spent fuel pools at the No. 4 reactor of the Fukushima Dai-ichi complex, but Japanese officials denied it. Hajime Motojuku, spokesman for plant operator Tokyo Electric Power Co, said the “condition is stable” at No. 4.

If Jaczko is correct, it would mean there’s nothing to stop the fuel rods from getting hotter and ultimately melting down. The outer shells of the rods could also ignite with enough force to propel the radioactive fuel inside over a wide area.

Jaczko did not say how the information was obtained, but the NRC and U.S. Department of Energy both have experts at the complex of six reactors along Japan’s northeastern coast, which was ravaged by last week’s magnitude-9.0 earthquake and subsequent tsunami.

As international concern mounted, the chief of the U.N. nuclear agency said he would go to Japan to assess what he called a “serious” situation and urged Tokyo to provide better information to his organization.

Japanese officials raised hopes of easing the crisis, saying early Thursday that they were close to completing a new power line that could restore the reactors’ cooling systems.

Naoki Tsunoda, a spokesman for TEPCO, said the new power line to the Fukushima Dai-ichi plant was almost finished and that officials planned to try it “as soon as possible,” but he could not say exactly when.

The new line could revive electric-powered pumps, allowing the company to maintain a steady water supply to troubled reactors and spent fuel storage ponds, keeping them cool. The company is also trying to repair its existing disabled power line.

The tsunami destroyed the complex’s backup power system and left operators unable to properly cool nuclear fuel. The 180 emergency workers have been working in shifts to manually pump seawater into the reactors.

Officials are also taking increasing criticism for poor communication about efforts at the complex. There has been growing unease at the U.N.‘s International Atomic Energy Agency’s 35 board member nations, who have complained that information coming from Japan on the rapidly evolving nuclear disaster is too slow and vague.

IAEA head Yukiya Amano spoke of a “very serious” situation and said he would leave for Tokyo within a day.

He said it was “difficult to say” if events were out of control, but added, “I will certainly have contact with those people who are working there who tackled the accident, and I will be able to have firsthand information.”

The nuclear crisis has partly overshadowed the human tragedy caused by Friday’s 9.0-magnitude earthquake, one of the strongest recorded in history.

Millions of Japanese have been with little food and water in heavy snow and rain since Friday. In some towns, long lines of cars waited outside the few open gas stations, with others lined up at rice-vending machines.

National broadcaster NHK showed mammoth military helicopters lifting off Friday afternoon to survey radiation levels above the nuclear complex, preparing to dump water onto the most troubled reactors in an effort to cool them down.

The defense ministry later said those flights were a drill – then later said it had decided against making an airborne drop because of the high radiation levels.

“The anxiety and anger being felt by people in Fukushima have reached a boiling point,” the governor of Fukushima prefecture, Yuhei Sato, fumed in an interview with NHK. He criticized preparations for an evacuation if conditions worsen, and said centers do not have enough hot meals and basic necessities.

Wednesday’s radiation spike was believed to have come from the complex’s No. 3 reactor. But officials also acknowledged that they were far from sure what was going on at the four most troubled reactors, including No. 3, in part because high radiation levels made it difficult to get very close.

While white smoke was seen rising Wednesday above No. 3, officials could not ascertain the source. They said it could be spewing from the reactor’s spent fuel pool – cooling tanks for used nuclear rods – or may have been from damage to the reactor’s containment vessel, the protective shell of thick concrete.

Masahisa Otsuki, an official with TEPCO, said officials are most concerned about the spent fuel pools, which are not encased in protective shells.

“We haven’t been able to get any of the latest data at any spent fuel pools. We don’t have the latest water levels, temperatures, none of the latest information for any of the four reactors,” he said.

Elevated levels of radiation were detected well outside the 30-kilometer emergency area around the plants. In Ibaraki Prefecture, just south of Fukushima, officials said radiation levels were about 300 times normal levels by late morning. It would take three years of constant exposure to these higher levels to raise a person’s risk of cancer.

A little radiation was also detected in Tokyo, triggering panic buying of food and water.

Given the reported radiation levels, John Price, an Australian-based nuclear safety expert, said he saw few health risks for the general public so far. But he said he was surprised by how little information the Japanese were sharing.

“We don’t know even the fundamentals of what’s happening, what’s wrong, what isn’t working. We’re all guessing,” he said. “I would have thought they would put on a panel of experts every two hours.”

Chief Cabinet Secretary Yukio Edano said the government expects to ask the U.S. military for help, though he did not elaborate. He said the government is still considering whether to accept offers of help from other countries.

There are six reactors at the plant. Reactors 1, 2 and 3, which were operating last week, shut down automatically when the quake hit. Since then, all three have been rocked by explosions. Compounding the problems, on Tuesday a fire broke out in No. 4’s fuel storage pond, an area where used nuclear fuel is kept cool, causing radioactivity to be released into the atmosphere.

Reactors 4, 5 and 6 were shut at the time of the quake, but even offline reactors have nuclear fuel – either inside the reactors or in storage ponds – that need to be kept cool.