ROCKVILLE, Md. — Despite repeated assurances that American nuclear plants are better equipped to deal with natural disasters than their counterparts in Japan, regulators said Thursday that recent inspections had found serious problems with some emergency equipment that would have made it unusable in an accident.

In addition, the staff of the Nuclear Regulatory Commission acknowledged that the agency’s current regulations and disaster plans did not give enough consideration to two factors that had greatly contributed to the continuing Fukushima Daiichi crisis in Japan: simultaneous problems at more than one reactor and a natural disaster that disrupts roads, electricity and other infrastructure surrounding a plant.

The briefing was part of a review requested by the commissioners to evaluate the vulnerability of American reactors to severe natural disasters like the ones that hit the Japanese plant in March.

Marty Virgilio, the deputy executive director of the agency, told the five commissioners that inspectors checked a sample of equipment at all 104 reactors and found problems at less than a third of them. The problems included pumps that would not start or, if they did, did not put out the required amount of water; equipment that was supposed to be set aside for emergencies but was being used in other parts of the plants; emergency equipment that would be needed in case of flood stored in places that could be flooded; and insufficient diesel on hand to run backup systems.