Nuclear reactors in Japan could still be operating up to 60 years after they were built, the government said Wednesday as it unveiled plans to boost their maximum lifespan by 50 percent.

The announcement comes as all but five of the country's 54 reactors are out of action, with the public demanding safety checks following the disaster at the Fukushima plant.

"The extension will be exceptionally approved when the safety of a plant is ensured," Chief Cabinet Secretary Osamu Fujimura told reporters. "But there is no change to the basic 40-year limit."

Presently reactors are licensed for 30 years and operators are allowed to apply for a 10-year extension to their lifespan.

Critics of nuclear power leapt on the announcement, which they said was irresponsible in the wake of the massive March 11 earthquake and tsunami that crippled the Fukushima Daiichi nuclear power plant.

"It is absolutely incredible," said Junichi Sato, director of environmental group Greenpeace Japan.

"This is an unacceptable risk for the people of Japan, especially when they are already suffering ongoing effects from the triple meltdown at Fukushima Daiichi."

The government is considering following the example of the United States where nuclear reactors can extend their operations by 20 years beyond a 40-year limit, if approved by federal authorities, a senior official said on Tuesday.

Toru Ogino of the Cabinet Secretariat also told Japanese media: "We are following a global trend."

The government is drawing up bills to tighten nuclear safety regulations following the Fukushima meltdowns, the worst nuclear accident since the 1986 Chernobyl disaster.