WASHINGTON - US President Barack said on Thursday he had ordered a comprehensive review of US nuclear facilities and said radiation from an earthquake-stricken power plant in Japan posed a "substantial risk" to people nearby.

Obama expressed confidence that Japan would recover from the tsunami and nuclear crisis that have seemingly overwhelmed its government, in remarks in the White House Rose Garden.

He said 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.

"We are working aggressively to support our Japanese ally at this time of extraordinary challenge," he said. "The US military, which has helped to ensure the security of Japan for decades, is working around the clock. To date we've flown hundreds of missions to support the recovery efforts."

Special Coverage:   Earthquake hits Japan

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.

Jaczko's agency will carry out the review Obama requested.

"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 said.

Obama, who is leaving for a trip to South America on Friday, stopped by the Japanese Embassy earlier in the day to sign a condolence book and said there that his administration felt "great urgency" to help Japan.

While providing that help, the US government is encouraging Americans in Japan to leave. It has sent charter planes to evacuate US citizens and family of embassy and military personnel.

Differing views over the danger zone around the stricken plant have divided the governments of the two countries.

The State Department recommended that US citizens within 50 miles (80 km) of the plant leave or stay indoors.

Japan's government has asked people living within 12 miles (20 km) to evacuate and those between 12 miles and 18 miles (30 km) to stay indoors.

"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."

The White House said Obama was confident Japan was aware of the severity of the crisis it faced.