VIENNA (Reuters)—U.S. regulators see room to improve safety at the country's nuclear power plants even though tests after the Fukushima disaster have shown they are fundamentally sound, the top U.S. watchdog said Tuesday.

More than two-thirds of the way through a 90-day task force review to see if the Japanese catastrophe had exposed any issues that needed quick action, a mixed picture was emerging, U.S. Nuclear Regulatory Commission Chairman Gregory Jaczko said.

"At this point we continue to believe that in the United States the plants operate safely and that the system we have is a robust safety system, but we do believe that there are areas where we can make improvements," he told reporters in Vienna.

He cited dealing with extended losses of electrical power, accounting for spent fuel pools during crises, emergency planning and properly addressing hazards such as earthquakes, hurricanes, tornadoes and other natural disasters.

The United States has 104 of the world's 440 reactors.

Japan's nuclear crisis, spawned by a March 11 earthquake and tsunami that crippled the Fukushima plant, has brought the safety issue to a head and prompted the U.S. safety review, results of which will be ready in around three weeks.

The U.S. survey looks not just at safety standards, but also at whether they are being implemented in the right way, he said on the sidelines on a conference hosted by the International Atomic Energy Agency.

"Once we complete that review, we likely will have changes to our requirements and will expect all the plants to comply with those changes as appropriate," he said.

U.S. regulators agreed this month to delay handling PG&E Corp.'s request to extend licenses of its Diablo Canyon reactors in California to give the utility more time to conduct a seismic study.

Mr. Jaczko said the discovery of a new fault near Diablo Canyon did not seem to be a major issue.

"Right now what we have seen is that the impact of that new fault appears to be consistent with the way...the utility has analyzed the plant in the past," he said.

Mr. Jaczko hailed IAEA proposals to improve safety and regain public trust by introducing random spot checks of nuclear plants worldwide but said it was important to cast a tighter net.

"The most important issue here is the recognition that there are a large number of nuclear power plants in the United States and of course IAEA couldn't be involved in doing inspections or peer review of some type at all of those facilities," he said.

"There has to be a way to narrow down the number of facilities that you would look at."