"Another possible risk would be energy prices could go up and possibly dependence on the Middle East or other fossil fuel exporting countries," Suzuki said.

"Having a vision of being nuclear energy-free is one thing. How to achieve it is another thing. It is very difficult to phase out nuclear power in a real sense," he said.

Japan's Prime Minister Naoto Kan bows in front of the Japanese national flag at the start of his news conference at his official residence in Tokyo July 13, 2011. [Photo/Agencies]

Economics Minister Kaoru Yosano, an ardent supporter of nuclear power, echoed concerns about economic costs, telling a news conference that substituting fossil fuel for atomic energy would be equivalent to a large rise in Japan's corporate tax and slice several trillion yen (tens of billions of dollars) off gross domestic product.

But he gave no time frame for that prediction.

Energy experts said Japan would have to boost fuel imports to make up for any short-term decline in nuclear power.

"It takes time to reduce reliance on nuclear power and replace it with alternatives, maybe a span of 10 years or so," said Koki Ota, senior economist at Sumitomo Shoji Research Institute, adding that demand for low-sulphur waxy residue and gas imports would strengthen the most.

Kan, who has come under fire for his handling of the nuclear crisis, defended his introduction of stress tests for reactors to soothe public safety concerns but apologised once again for the apparent abruptness of the move.

He said that idled reactors that completed the first stage of the two-stage stress tests could resume operations if experts and relevant cabinet minister agreed.

Last week's decision to introduce the tests, simulations to confirm nuclear plants' safety and check their ability to withstand extreme events, fanned corporate worries about power shortages if idled reactors stay off-line, and outraged some local officials who had been ready to approve restarts after earlier government safety assurances.  

Kan said he was not thinking about calling a snap election over energy policy and sidestepped a question on when he would quit.

Among the conditions he has previously cited for resigning is passage of a bill that would promote renewable energy sources such as solar and wind power.