TOKYO - Japan could give no timetable Monday for restarting 35 nuclear plants idled in the wake of a tsunami-triggered meltdown crisis, with an official saying resuming operations depends on safety "stress tests" sometime in the future.

The news that the electricity shortages caused by the shutdowns might drag on came as Japan marked the fourth month after the March 11 earthquake and tsunami decimated much of northeastern Japan and triggered a meltdown crisis at the country's Fukushima Dai-ichi nuclear plant.

The government ordered safety checks on all of Japan's 54 nuclear reactors following the nuclear disaster - the worst nuclear crisis since Chernobyl. Only 19 reactors are currently operating, causing electricity shortages amid sweltering heat.

Trade ministry official Yoshifumi Ohno said 35 idled nuclear reactors will undergo the new safety tests, known as "stress tests." The initial stage of the tests will examine whether those reactors can withstand big earthquakes and tsunami.

The March earthquake and tsunami knocked out power at the Fukushima plant, sending it toward meltdown in a crisis that engineers are still struggling to contain. The plant operator has come under heavy criticism for failing to sufficiently prepare for the disasters.

Ohno said the second stage of the "stress tests" will be more comprehensive, and apply to all of Japan's nuclear reactors. But he gave no further details, and did not say when the "stress tests" would begin.

Nuclear power generates about 30 percent of Japan's electricity. If nuclear reactors currently shut for maintenance face delays in resuming operations, the country could face a power shortfall in the months ahead.

With the loss of the Fukushima plant, which used to supply power to Tokyo areas, the government recently imposed energy restrictions on companies, factories and shopping malls to cope with electricity shortages.

The energy limit does not apply to households, but the government has urged them to conserve energy.

The government has also launched a campaign urging office workers to shed their suits and ties for lighter clothes such as polo shirts to endure the country's summer heat.