Japan's government on Saturday approved bringing the country's first nuclear reactors back online since last year's earthquake and tsunami led to a nationwide shutdown, going against wider public opinion that is opposed to nuclear power after Fukushima.

The decision paves the way for a power company in western Japan to immediately begin work to restart two reactors in Ohi town, a process that is expected to take several weeks.

Despite lingering safety concerns, the restart could speed the resumption of operations at more reactors across the country. All Japan's 50 nuclear reactors are offline for maintenance or safety checks.

Public opposition to the resumption of nuclear operations remains high because of the crisis the tsunami touched off at Fukushima Dai-ichi plant, the worst atomic disaster since Chernobyl. As the government announced its decision, a protest was held outside the prime minister's offices.

The restart is being closely watched as an indicator of how aggressively the government will act to approve operations as soon as possible to avert power shortages as demand increases during the summer months. It says the reactors in the town of Ohi are particularly important because they are in an area that relied heavily on nuclear before the crisis, and have passed safety checks.

"Safety is our main concern," said trade and industry minister Yukio Edano. "We have approved the beginning of the restarting process. It will take some time for the reactors to begin generating electricity."

He said the government would request people continue to save energy, and added that if there are safety problems, the process could be delayed.

Prime Minister Yoshihiko Noda announced the government's approval after Ohi's mayor and the local governor publicly stated they support the plan. Local approval isn't needed legally.

"I approve the plan because I have been assured of the government's safety efforts and because it will provide stability for our industries," Issei Nishikawa, the governor of Fukui prefecture which oversees Ohi, said after meeting Noda in Tokyo on Saturday.

Power crunch expected in the summer

Kansai Electric Power Co. officials say bringing the two reactors online is needed to help avert a power crunch in Osaka, Japan's second-largest metropolis, and other areas in the west. They say demand is expected to peak in mid July or early August, so they need to begin work immediately to get the reactors up and running to avoid shortages.

KEPCO says it will take about three weeks to get the first reactor back online. It will start the plant's No. 3 reactor first, and the No. 4 reactor after that.

Last year's March 11 disaster devastated the Fukushima plant, leading to explosions, meltdowns and massive leaks of radioactive material. Tens of thousands of residents near the plant were forced to evacuate, and large areas around the reactors there are still off limits because of the danger of exposure.

The government and power company that ran the plant, Tokyo Electric Power Co., have acknowledged they were unprepared for such a disaster. They are in the process of reviewing and strengthening measures to make sure that such a disaster does not recur.

But while safety concerns still run deep, Saturday's decision could speed up efforts to get more plants running across the country.

Until now, the government had withheld approval to restart any nuclear reactors, and the last reactor went offline in May.

Before the crisis, nuclear generated about one-third of Japan's electricity.

Japan is debating renewable energy targets of between 25 per cent to 35 per cent of total power generation by 2030. But the sudden shutdown of nuclear plants has hit Japan's economy hard.

To offset the shortfall, utilities have ramped up oil- and gas-based generation, and that contributed to the country's biggest annual trade deficit ever last fiscal year.