Japanese regulators said Monday that two ageing nuclear reactors can stay on line for up to 20 more years -- the first such exception under tighter safety rules imposed after the 2011 Fukushima crisis.

Environmental group Greenpeace criticised the decision, saying earthquake risks were being ignored.

Japan shut down dozens of reactors after a magnitude 9.0 earthquake-generated tsunami on March 11, 2011 triggered meltdowns at the Fukushima nuclear plant in the northeast, the world's worst nuclear accident since Chernobyl in 1986.

The ensuing decrease in nuclear power generation forced resource-poor Japan to turn to pricey fossil fuels. Prime Minister Shinzo Abe has declared that nuclear power is essential for the economy as he pushes to get reactors back in operation.

The Nuclear Regulation Authority (NRA) said the No. 1 and No. 2 reactors at Kansai Electric's Takahama plant can operate for up to 20 more years because they meet safety guidelines.

The utility had asked the nuclear watchdog to extend the operational period of the reactors. Both are over 40 years old, normally the maximum period under NRA rules.

The reactors have been switched off since 2011.

The move comes after a district court in March issued an injunction ordering a temporary shutdown of the No. 3 and No. 4 reactors at the same plant.

The Takahama facility is located 350 kilometres (215 miles) west of Tokyo.

Monday's decision was quickly denounced by Greenpeace, which said the move "goes far beyond regulatory failure".

"The NRA is... doing everything it can to ignore the earthquake risks to nuclear plants in Japan," Kendra Ulrich, senior global energy campaigner at Greenpeace Japan, said in a statement.

Ulrich, citing "known seismic risks" in the area, added: "The NRA is showing itself to be incapable and unwilling to protect the people of Japan."

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THE KANSAI ELECTRIC POWER CO