WASHINGTON (AFP) – President Barack Obama said Tuesday he was "deeply worried" about the potential human cost of quake-hit Japan's nuclear crisis and vowed to "further improve" the safety of US atomic facilities.

"Nuclear plants are designed to withstand certain levels of earthquakes, but having said that, nothing's completely failsafe, nothing is completely foolproof," he said in an interview with a CBS television station in Pennsylvania.

"So each time these kinds of events happen, I think it's very important for us to examine how we can further improve the safety and performance of these plants," said the president.

But he emphasized that the US Nuclear Regulatory Commission "thinks through all eventualities" as part of its oversight of US atomic power and noted "all energy sources have their downside" -- citing the catastrophic Gulf of Mexico oil spill of 2010.

"I do think it's important for us to think through constantly how can we improve nuclear technologies to deal with additional safety concerns that people have," he said.

The US government has thus far rebuffed relatively muted calls for a nuclear moratorium amid the crisis at Japan's Fukushima power plant, which has suffered explosions and a fire in the wake of Friday's devastating earthquake and tsunami.

Obama said he was not worried that any radiation seeping out the plant might reach US shores, but emphasized: "I'm deeply worried about radiation effects in Japan."

"Our hearts go out to the people of Japan. They are dealing with a triple whammy -- the earthquake, the tsunami, and now this nuclear accident. So we're providing them all the support that we can. We want to make sure that they know that we have their backs and are one of our closest allies and closest friends."

"There are some dangers for radiation release that could affect the immediate vicinity of nuclear plants and potentially could drift over other parts of Japan," he told KDKA.

"But I've been assured that it -- any nuclear release dissipates by the time it gets even to Hawaii, much less the mainland of the United States," he said.