TOKYO—Japan's two-stage test of the stability of its nuclear plants in the event of a natural disaster meets global safety standards, but more work is needed to prepare for the aftermath of any nuclear accident, the International Atomic Energy Agency said Tuesday.

In a report that could further delay the nation's push to restart idled reactors, the IAEA said that, overall, the tests by Japan's main nuclear regulator "are generally consistent with IAEA Safety Standards."

An...