KASHIWAZAKI, Japan — Japanese regulators discovered a fresh leak of radioactive material today from a nuclear power plant damaged in an earthquake this week, news reports said, adding to criticism of the embattled plant operator.

Nuclear inspectors probed the Kashiwazaki-Kariwa plant, which suffered a barrage of leaks and malfunctions in Monday's 6.8-magnitude quake in northwestern Japan. The plant was ordered shut down indefinitely on Wednesday.

The Nuclear and Industrial Safety Agency found radioactive iodine had leaked from an exhaust pipe at the plant, Kyodo News Agency and national broadcaster NHK reported. Operator Tokyo Electric Power Co. already had announced the release of other radioactive materials from the exhaust vent previously.

NHK said the material had been leaking into the atmosphere until Wednesday.

The inspectors, however, concluded the leak was too small to harm the environment or public health, the reports said. Officials at the agency said they could not immediately confirm the reports.

Also today, TEPCO announced that the force of the quake had exceeded its resistance guidelines at all seven reactors, sometimes by more than double. NHK reported that the reading at the No. 1 reactor was the strongest quake ever measured at a Japanese reactor.

Members of a separate panel, the Nuclear Safety Commission, toured the Kashiwazaki-Kariwa plant today and regaled TEPCO for missteps in its response to Monday's quake, which killed at least 10 people and injured more than 1,000 others.

Still, the commission concluded none of the errors threatened public health.

"The safety of ... (the) plant was fundamentally maintained and we avoided the serious consequences of a nuclear accident," Commission Chairman Atsuyuki Suzuki said in a statement. "The list of problems announced by TEPCO have no serious effect on the safety of the reactor."

Commission members criticized TEPCO for a bungled response to a quake-triggered fire at an electrical transformer. Plant officials said they had no chemical fire vehicle at the plant, and local fire officials took 90 minutes to respond to their call.

The commission also said the fuel rods in the plant were stable, but that the inside of the reactors should be checked more thoroughly.

The plant fire, which blazed for two hours, likely broke out after the quake caused the ground beneath an electric transformer to sink, damaging cables, causing a short circuit and igniting leaked insulation oil, the business daily Nikkei reported, citing a probe by the prefectural government.