Engineers at a stricken nuclear plant in Japan have connected an electricity line to one of the reactors and are working to restore power for its cooling system, officials said today.

The cable has been linked to reactor No. 2 at the troubled Fukushima No. 1 plant, whose operator TEPCO has been battling to prevent a full-blown meltdown following a massive earthquake and tsunami.

There are six reactors at the plant, which is located about 250 kilometres northeast of Tokyo and has already leaked radiation, prompting the government to order an evacuation within a 20-kilometre radius.

"The line itself was connected to the reactor No. 2 but electricity has not been restored yet," said a spokesman for the nuclear safety agency, Fumiaki Hayakawa.

"If the power is turned on without checks it may malfunction. They are checking the facility now. If no problem is found at the facility today, the power will resume as early as tomorrow."

Once power is back up, the radiation-suited Fukushima engineers hope they can get vital cooling systems online. In the meantime, they have been dumping water by hose and by air on the reactors to cool the fuel rods.

Given the extent of damage at the plant, it is unclear whether the cooling system will work even if power is restored.

"Although we are doing our best, unfortunately we cannot say when electricity will be restored," said a TEPCO official.

Reactors 1 and 2 usually share the same electricity line, so the cable could in theory restore power to both if the system still functions.

Four of the plant's six reactor units -- numbers one to four -- have been in danger of spewing dangerous amounts of radioactivity, following a series of hydrogen explosions and fires at buildings housing the troubled reactors.

A 9.0-magnitude earthquake last week, followed by monster tsunami waves and aftershocks, knocked out the power supply, including generators for emergency use, at the plant on the Pacific coast.