Japan may go "momentarily" without nuclear power next month when the only one reactor still in operation shuts down for maintenance work, the country's industry minister warned Sunday.

Yukio Edano made the comment as the government prepared to restart two offline nuclear reactors amid criticism from media and environmental groups sceptical over the safety of atomic power after the Fukushima accident.

"The number of nuclear reactors operating across the country may go down to zero, perhaps momentarily, from May 6," he said in a seminar in Tokushima, western Japan.

The government of Prime Minister Yoshihiko Noda announced on Friday that it was safe and necessary to restart the reactors at the Oi nuclear power plant in western Japan, which could help prevent power shortages in the summer months.

Only one of Japan's 54 reactors -- in northernmost Hokkaido -- is in operation at present, but it is scheduled to be shut down for maintenance work on May 5.

But it was not certain if and when the government could gain approval from regional authorities around the Oi plant for the reactors to be restarted amid persistent public distrust.

"Without nuclear reactors, it is understandable that there will be considerable strain on many areas in this summer," Edano added.

Edano on Saturday called on the governor of Fukui, where the Oi plant is located. The governor, Issei Nishikawa, did not give an immediate response to his request for approval of the plan.

But the major daily Mainichi Shimbun said Sunday: "It is hard to understand why the government is in such a haste to restart the reactors."

It added in an editorial that more thorough checks were needed to ensure safety.

"Independent studies show that there will be no power shortages," said Wakao Hanaoka, the Japan campaign manager for the environment watchdog Greenpeace.

A massive earthquake and tsunami in March last year caused reactor meltdowns at the Fukushima Daiichi plant. None of reactors shut for regular checks before the disaster have resumed operation amid safety concerns.

"The nuclear industry and the government were totally unprepared for the disaster at Fukushima Daiichi and now they are trying to pretend they can call Oi safe without improving safety or emergency measures," Hanaoka said.

The government set criteria nine days ago for restarting nuclear reactors included measures to prevent a nuclear accident even if reactors are hit by natural disasters as severe as those that ravaged the Fukushima plant.

"It is uncertain if the plan will ever gain an understanding of communities which have raised objections to the resumption of the reactors," the Asahi Shimbun reported Sunday.

The influential daily criticised the Noda administration for being "inconsistent" over its nuclear power policy.

Before Noda took office last September, he promised to follow his predecessor Naoto Kan in ridding Japan of nuclear power, Asahi said.

But he backtracked last January when he said in a policy speech that the resources-poor country would reduce its dependence on nuclear power "as much as possible on a medium- and long-term basis".

Striking a more positive tone, the conservative daily Sankei Shimbun called on Noda to visit Fukui himself and "speak clearly in his own words about his government's energy policy and why it is necessary to restart reactors".