Companies that operate U.S. nuclear power plants are not reporting some equipment defects that could create safety risks, according to a new report.

The inspector general at the Nuclear Regulatory Commission also raised questions in the report about the agency's oversight, saying reporting guidelines for the nuclear industry are "contradictory and unclear."

Thursday's report arrives as Japan grapples with a nuclear crisis that began two weeks ago with a massive earthquake that triggered a violent tsunami. Japan's Fukushima Dai-ichi nuclear complex was hit by both, causing it to spew radiation into the environment and to spark fears of widespread contamination.

The U.S. military is conducting four evacuation flights out of Japan Thursday, bringing about 600 military dependents to the Seattle and Denver airports. About 3,600 dependents have been flown out so far. About 13,000 U.S. Naval dependents live within 200 miles of the reactor

While U.S. officials have been emphatic in assuring that the nation's 104 nuclear plants are safe in the wake of the Japan crisis, the report now casts doubt on those claims.

NRC inspectors found at least 24 instances where possible equipment defects were identified but not reported to the agency from December 2009 through September 2010, the report said.

Unless the NRC takes steps to improve its reporting guidelines, "the margin of safety for operating reactors could be reduced," the report said.

The NRC sought to minimize the report by saying the findings focused on manufacturing defects that both utilities and NRC inspectors have processes for identifying and reporting.

"The fundamental issue identified by the report is administrative and pertains to how these defects are reported," the NRC said in a statement to FoxNews.com. "The NRC has a variety of other regulations that effectively encompass reporting all defects, and the NRC continues to conclude plants are operating safely."

The NCR added that it will look at the report "to see if our reporting systems can be further strengthened."

Rep. Ed Markey, D-Mass., the top Democrat on the National Resources Committee, said he will be seeking further answers from the NRC and its inspector general regarding the potential public safety hazards and will be pursuing remedies.

"This troubling study by the NRC's inspector general raises serious questions about the self-policing allowed at nuclear facilities with regard to reporting of safety concerns," he said in a statement to FoxNews.com. "While there are no specific examples listed in the report, it is apparent that confusion and omissions regarding the reporting of defects at nuclear facilities are commonplace."

Fox News' Jennifer Griffin and The Associated Press contributed to this report.