Still, without a timeframe for the tests, concerns remain about summer power shortages that could hurt the economy.

Last week's surprise announcement that the government would conduct stress tests alarmed corporate Japan and outraged some local authorities, who had been prepared to approve reactor restarts after receiving safety assurances from the government.

The first stage of the stress tests will target reactors which have already completed routine checks and are ready for restart. The checks will assess tolerance of severe phenomena exceeding those for which they were designed.

A second stage of tests will assess all of Japan's 54 reactors and involve a comprehensive safety assessment of all Japan's nuclear plants, the government added in its statement.

Four months after the Fukushima Daiichi plant was smashed by a tsunami and began leaking radiation, only 19 of the country's 54 reactors are running and if some do not resume operations, Japan could be without nuclear power by next April.

The disaster has also sparked a broader public debate about the role of nuclear energy in earthquake-prone, resource-poor Japan, which relied on atomic power for almost 30 percent of its electricity before the crisis.

"Safety and a sense of security are the top priority," Chief Cabinet Secretary Yukio Edano told a news conference.

"On the other hand, the government must fulfill its responsibility for a stable supply of electricity and is coordinating on this with relevant ministries ... and will make every effort to secure (supply) in the medium and long term," he added.