WASHINGTON (AP) — The top U.S. nuclear regulator said Monday he will not change a recommendation that U.S. citizens stay at least 50 miles away from Japan’s crippled nuclear power plant, even as he declared that the crisis in that country remains “static.”

Gregory Jaczko, the chairman of the Nuclear Regulatory Commission, acknowledged in an interview with The Associated Press that the month-old crisis in Japan has not yet stabilized. But he said conditions at the Fukushima Dai-ichi plant have not changed significantly for several days.

“We describe the situation as static but not yet stable,” Jaczko said.

“It hasn’t really changed too much in the last few days,” he added, but it will be weeks or even months before the plant is stabilized.

The March 11 earthquake and tsunami knocked out power at the Fukushima plant and reactors have been overheating ever since. In Japan on Tuesday, the Nuclear Safety Commission of Japan raised the severity rating of the crisis from 5 to 7, the highest level and on par with the 1986 Chernobyl disaster.

Progress in stabilizing the complex comes slowly most days, or not at all, as new tremors and radiation repeatedly halt work. A new aftershock Monday briefly cut electricity to the plant and halted work while technicians took cover, but did not endanger operations, according to Japanese officials.

The Japanese government, meanwhile, added five communities Monday to a list of places people should leave to avoid long-term radiation exposure. A 12-mile radius has been cleared around the plant already.

Jaczko said the most important job at the plant still is keeping water in the spent fuel pools to cool the highly radioactive fuel rods, reducing the threat of a meltdown and a catastrophic release of radiation.

Jaczko, who traveled to Japan last month, said the NRC has begun a two-pronged approach to review the safety of the 104 commercial U.S. nuclear reactors in the wake of the Japanese crisis. A 90-day review should be completed in June, with another report expected by the end of the year.

“We want this to be a very systematic and methodical review and make sure we identify all the important issues, and that we work with a sense of urgency and speed to address those issues in the appropriate way,” he said, adding that he expects the reviews to result in recommendations for significant regulatory changes.

“Fundamentally, I expect that there will be some things we will want to change and need to change as a result of what comes out of this 90-day review and longer-term review, based on events in Japan,” he said.

A task force made up of high-ranking NRC staff is conducting the two reviews, and the five-member commission will act quickly once the reports are released, Jaczko said.

On the 50-mile evacuation zone for U.S. citizens in Japan, Jaczko called his March 16 recommendation “prudent” and said it was based on projections for continued deterioration at the plant. The Japanese government had set a 12-mile evacuation zone, and the U.S. decision raised questions about U.S. officials’ confidence in Tokyo’s risk assessments.

“I’m still very comfortable” with the decision, Jaczko said.

Asked whether he set up a double standard — one for nuclear plants in foreign countries and another for U.S. plants, where a 10-mile evacuation zone is the current standard — Jaczko said no.

“I wouldn’t say that’s a contradiction,” he said, noting that the 10-mile U.S. evacuation zone refers to emergency planning prior to a nuclear disaster. If events warrant, a larger evacuation zone can be created.

“Ultimately, decisions about protective actions (in the event of a nuclear disaster) are made by state and local authorities,” he said, not the NRC.

On another topic, Jaczko said he believes spent fuel can be stored safely either in pools or in dry cask storage. Sen. Dianne Feinstein, D-Calif., sent Jaczko a letter Monday urging the NRC to establish regulations that would encourage plant operators to move more quickly to store spent fuel in dry casks, rather than in pools that must be kept cooled.

Feinstein cited a 2006 study by the National Research Council indicating that dry cask storage systems have inherent safety advantages over spent fuel pools.

Jaczko disputed that, saying both methods are safe.

The United States has not had an accident involving spent fuel in decades, and spent fuel at commercial U.S. reactors “continues to be safe and secure,” even without a designated site to store nuclear waste, Jaczko said. The Obama administration has abandoned plans for a nuclear waste dump in Nevada, prompting sharp criticism from some lawmakers in both parties.

Jaczko declined to speculate on whether the Japanese crisis would cause a slowdown in a planned expansion of U.S. nuclear reactors backed by President Barack Obama. Jaczko said the NRC has “a very robust system” to license reactors that takes into account a wide range of factors.

“Ultimately safety rests with the (plant operator),” he said. “It’s our job to make sure they get there.”

If the NRC considers plants unsafe, it will take corrective action, up to and including shutting down plants if necessary, Jaczko said.

Three U.S. nuclear power plants — in South Carolina, Kansas and Nebraska — need increased oversight from federal regulators because of safety problems or unplanned shutdowns. But Jaczko said all 65 U.S. nuclear plants in 31 states are operating safely.