All six reactors at Japan's troubled Fukushima Dai-ichi nuclear plant have been reconnected to an external power supply, while high radiation levels continued to be reported as far as 40 kilometres away.

Tokyo Electric Power Co. said Tuesday that progress was underway and lighting had been restored at the No. 3 reactor, bringing workers much closer to controlling the nuclear crisis and restoring the reactors' ability to cool.

"I think the situation will head toward a better direction, but it is too early to say that things have stabilized sufficiently," said Tokyo Electric Executive Vice President Sakae Muto.

However, the company said that workers still need to determine whether equipment has been damaged at the plant before it can turn on the reactors' cooling system. That could take days or weeks.

Hidehiko Nishiyama, spokesperson for the government's nuclear safety agency, said at a separate news conference that the restoration of electricity would allow officials to better monitor the situation inside the plant.

He also said at this point a large-scale meltdown is unlikely.

Meanwhile, there was growing evidence of nuclear fallout from the damaged plant, with radiation 400 times beyond the normal level found in soil 40 kilometers away, broadcaster NHK reported.

Tokyo Electric Power Company also found radioactive material in seawater near the plant's southern discharge canal, according to the UN's atomic energy agency.

On Monday, efforts to cool spent nuclear fuel pools had been hampered by clouds of smoke and steam at the No. 2 and No. 3 reactors.

In a reminder that the battle to stop the spread of radiation at the plant is still not over, officials said that a pool holding spent nuclear fuel at the No. 2 reactor had heated up to around the boiling point.

Spent fuel rods can generate intense heat and dangerous radiation. The rods need to be stored in a water-filled pool for a year or more to allow them to cool, and the heat from the pools needs to be constantly removed.

The worry at the overheating Fukushima fuel pool is that if it's not cooled down soon, radioactive steam will be emitted. And if the situation worsens, the water within could boil away, exposing the fuel rods, which would throw up much more radiation.

Earlier on Tuesday, work had to be halted because steam appeared to have risen from reactor No. 2 and white haze was detected above reactor No. 3, Kyodo News reported.

Radiation levels spiked briefly and engineers were told to leave the plant. Work resumed again shortly after dawn.

But emergency crews eventually managed to dump enough seawater onto an overheating storage pool at the No. 2 reactor to bring its temperature down to 50 degrees Celsius.

Now that technicians have attached power cables to all six reactors, the priority now is patching up the No. 2 reactor so that the electricity can be turned on.

Cooling pumps are once again working at reactor No. 5.

Toshiba said it had sent 100 engineers to help.

Australia has also sent a remotely operated water cannon system to help spray water on the plant.

Despite these latest setbacks, the U.S. Nuclear Regulatory Commission in Washington said Monday that the crisis appeared close to stabilizing.

An NRC official said Units 1, 2 and 3 have all seen damage to their reactor cores, but that containment is intact.

But the Vienna-based International Atomic Energy Agency said radiation seeping into the environment is still a concern and needs to be monitored.

"We are still in an accident that is still in a very serious situation," said Graham Andrew, senior adviser to IAEA chief Yukiya Amano.

IAEA monitoring stations have detected radiation 1,600 times higher than normal levels -- but only within the 20 kilometres radius of the evacuation area declared by the government last week.

Radiation levels drop dramatically the further you go from the nuclear complex.

With files from The Associated Press