Japan on Friday said nuclear power would remain an important source of energy as the government approved a new Basic Energy Plan, the first policy of its kind since the 2011 Fukushima disaster.

The government of conservative Prime Minister Shinzo Abe said atomic generation would resume once regulators were sure of the safety of each reactor, all of which were switched off after the tsunami-sparked catastrophe at the Fukushima plant.

The move comes despite widespread public unease over a technology once almost unquestioningly accepted as necessary in resource-poor Japan. Polls suggest more than half of the population is against restarting the reactors.

The last Basic Energy Plan, approved in June 2010, was more focused on cutting CO2 emissions, saying Japan would aim to increase the use of nuclear and renewable energy to more than 50 percent by 2020 and about 70 percent over the following decade.

The new plan says the country will "reduce" the use of nuclear power to a level "as low as possible" by increasing the percentage of renewable energy, but it did not give any numerical targets or dates.

"The government is starting over, withdrawing the energy strategy drawn up before the disaster. This is where we started," said Chief Cabinet Secretary Yoshihide Suga.

When the huge tsunami of March 2011 smashed into Japan, it swamped cooling systems at the Fukushima Dai Ichi nuclear plant, sending reactors into meltdown and spewing radiation over a large area, forcing the evacuation of hundreds of thousands of people.

More than 18,000 people were killed in the tsunami-quake disaster. Three years on, around 140,000 are still unable to return to their homes around the Fukushima plant.

The nuclear facility remains crippled, with a clean-up operation there expected to last decades.

Environmentalists on Friday lashed out at the new energy plan, which they said was "a product of compromise" intended to give succour to the nuclear industry.

"Japan's new Basic Energy Plan which the Cabinet agreed on today is a product of compromise between the government and politicians, and it is as close as 'basic business support plan' for the utilities and nuclear industry," said Hisayo Takada, Greenpeace Japan's climate and energy campaigner.

"We all know that a nuclear power plant accident can be a fateful crisis for the country," she said. "It is totally unacceptable to keep using such a... dangerous electricity generation method for another 20 years."