Japan says it is stepping up efforts to cool overheating fuel at the tsunami-hit Fukushima Daiichi nuclear plant.
Helicopters and water cannon have dumped seawater to try to prevent fuel rods melting, as engineers try to restore power to the coolant pumps.
The International Atomic Energy Agency says the situation remains serious but has not significantly worsened.
The confirmed death toll from Friday's 9.0 magnitude quake, which triggered the tsunami, has risen above 5,400.
Police say about 9,500 people are still missing.
IAEA official Graham Andrew told reporters in Vienna that the situation at Fukushima had not deteriorated, but could yet do so.
"We could say it's reasonably stable at the moment compared to yesterday," he said.
Mr Andrew said reactor 4, which was inactive but being used to store spent fuel rods, was still a "major safety concern" and that water levels were also falling in the cooling pools of reactors 5 and 6.
But he said there had been no rise in radiation in Tokyo and that levels remained far below those dangerous to human health.
The head of the IAEA, Yukiya Amano, is en route to Tokyo to be briefed by Japanese officials.
Despite assurances, the streets of the capital - including its popular Ginza retail district - remain unusually empty, with many people continuing to stay at home or leaving the capital.
Japan's military CH-47 Chinook helicopters began spraying tonnes of seawater on reactors 3 and 4 at Fukushima, 220km (140 miles) from Tokyo, at 0948 local time (0048 GMT), officials said.
The aircraft dumped four loads before leaving the site in order to minimise the crews' exposure to radiation. On Wednesday, the helicopters were forced to abort a similar operation amid concerns over high radiation levels.
The BBC's Chris Hogg in Tokyo says the helicopters can carry an enormous amount of water but given the high winds it is difficult to know whether it has been dropped accurately.
Video footage suggests the attempts were not very successful, with most of the water falling outside the target buildings.
Later military lorries on the ground joined in with water cannon, dousing reactor 3.
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, Japan's NHK TV reported.
The operation was intended to help cool the reactors and also to replenish water in a storage pond with spent fuel rods.
The Tokyo Electric Power Company (Tepco), which runs the plant, said it had started work to connect a 1-km (0.6-mile) electricity cable from the main grid to the plant.
The atomic crisis was triggered when the facility's power supply and back-up generators were crippled by the natural disaster.
"If the restoration work is completed, we will be able to activate various electric pumps and pour water into reactors and pools for spent nuclear fuel," a spokesman for Tokyo Electric Power Company, which runs the plant, told the AFP news agency.
The US has also been asked to fly a drone over the site to help assess the situation.
The crisis has prompted China to suspend approval of new nuclear power stations and carry out checks on existing reactors.
China currently gets about 2% of its electricity from nuclear power, but is building more reactors than any other country in the world.
Britain has advised its nationals currently in Tokyo and to the north of the capital to consider leaving the area, and to keep outside an 80km radius of the Fukushima plant, in line with US state department instructions.
France has urged its citizens in Tokyo to leave the country or move south. A French air force jet took 250 French nationals to South Korea, and two Air France planes are due to begin evacuations.
In areas of the north-east badly hit by the tsunami, bitter winter weather has added to the misery of survivors, though more supplies are now reported to be reaching them.
Japan's Kyodo news agency quoted rescuers as saying that the search for victims had expanded over a wider area as access had improved with the clearance of debris.
The number of people now known to have died in the twin disaster stands at 5,692 with 9,506 people listed as missing.
But Kyodo reports that the official toll is based on names registered with police, and that the true figure could be in the tens of thousands.
About 380,000 people are currently still in temporary shelters, many sleeping on the floor of school gymnasiums.
The crisis has also continued to affect the markets - the benchmark Nikkei index fell 3.6% in early Thursday trading in Tokyo, shortly after the yen briefly hit the highest level against the US dollar since World War II.