Washington: President Barack Obama said on Thursday he had ordered a comprehensive review of US nuclear facilities, maintaining his support for atomic energy while seeking to apply lessons from the crisis in Japan.

Obama expressed confidence that Japan would recover from the earthquake, tsunami and nuclear emergency that have seemed to overwhelm its government, but said radiation from a stricken plant there posed a “substantial risk" to people nearby.

He pledged to support Japan while Washington also seeks to aid and evacuate Americans from the country.

“In the coming days, we will continue to do everything we can to ensure the safety of American citizens and the security of our sources of energy," he told reporters at the White House. “And we will stand with the people of Japan as they contain this crisis, recover from this hardship, and rebuild their great nation."

US Nuclear Regulatory Commission chairman Gregory Jaczko said the United States was working to provide ideas and possibly equipment to help Japan cool its overheating Daiichi nuclear power plant about 240 km (150 miles) north of Tokyo.

He stressed that it could take weeks to succeed in cooling the reactor down.“This is something that will likely take some time to work through, possibly weeks, as eventually you remove the majority of the heat from the reactors and then the spent fuel pool," Jaczko told a briefing at the White House.

His agency will carry out the US review Obama requested. The commission will meet on Monday to begin discussions about it, a NRC spokesman said.

The US nuclear industry said it was taking steps to protect the country’s nuclear plants from a catastrophe like the one in Japan. Nuclear energy provides about 20% of US electricity.

“Our nuclear power plants have undergone exhaustive study, and have been declared safe for any number of extreme contingencies," Obama said. “But when we see a crisis like the one in Japan, we have a responsibility to learn from this event, and to draw from those lessons to ensure the safety and security of our people."

Obama stressed that the United States did not expect harmful radiation to reach its shores or territories and told Americans they did not need to take precautions other than staying informed.

Condolences, evacuation

The president, who leaves for South America on Friday, signed a condolence book at the Japanese Embassy and said his administration felt “great urgency" to help.

While providing that help, the US government has sent charter planes to evacuate US citizens and relatives of embassy and military personnel.

The two countries’ governments have offered differing views over the danger zone around the plant.

The State Department recommended that US citizens within 50 miles (80 km) of the plant leave or stay indoors. Tokyo has asked people living within 12 miles (20 km) to evacuate and those between 12 miles and 18 miles (30 km) to stay indoors.

Obama referred to that difference without criticizing the Japanese government.

“Even as Japanese responders continue to do heroic work, we know that the damage to the nuclear reactors in Fukushima Daiichi plant poses a substantial risk to people who are nearby," Obama said.

“That is why yesterday we called for an evacuation of American citizens who are within 50 miles of the plant. This decision was based upon a careful scientific evaluation."

US officials said they were continuing to help evacuate Americans who want to leave the reactor area and were sending 14 buses to an area north of Sendai.

The Pentagon announced plans for a voluntary evacuation of US military families from Honshu, Japan’s largest island. A spokesman for US forces in Japan estimated that around 20,000 dependents would be eligible.