Tokyo ( CNN ) -- Even as Washington and Tokyo disagreed on the extent of the threat a damaged nuclear power plant poses , President Barack Obama told the Japanese prime minister Thursday that the United States will help Japan rebuild following last week 's devastating earthquake and tsunami . The two leaders had a 30-minute phone call at 10:30 a.m. Thursday ( 9:30 p.m . ET Wednesday ) . During the phone call with Prime Minister Naoto Kan , Obama voiced sympathy for Japan 's plight . `` The president again conveyed his deep condolences at the tragic loss of life and the widespread suffering in northeastern Japan , '' a White House statement on the call said . `` The president emphasized that the U.S. is determined to do everything possible to support Japan in overcoming the effects of the devastating earthquake and tsunami that struck on March 11 . '' Kan also briefed the president on the status of Japanese efforts to contain the nuclear emergency at the Fukushima Daiichi plant in the country 's northeast , the White House said . The developments came as the nations established significantly different radiation exposure warning zones . The U.S. military will not allow troops to get within 80 kilometers ( 50 miles ) of the damaged plant . The Japanese government has told people to evacuate to at least 20 kilometers ( 12 miles ) away from the damaged reactors . Japanese Chief Cabinet Secretary Yukio Edano said Thursday it 's `` understandable '' that the United States would make a `` more conservative decision '' on the warning zone as it tries to protect its own citizens . He suggests that is , in part , because the United States is `` not directly controlling '' the situation . Meanwhile , a Tokyo Electric Power company official said Thursday that -- based on information gathered from a helicopter that flew over the Fukushima facility on Wednesday -- authorities believe that there is water in a key fuel pool outside one of the plant 's most troubled reactors . Hours earlier , Gregory Jaczko , the head of the U.S. Nuclear Regulatory Commission , told Congress that spent fuel rods in Unit 4 of the plant had been exposed , resulting in the emission of `` extremely high '' levels of radiation . Having water in the fuel pool is important because it helps absorb the radiation . The Fukushima complex lost its power Friday , after a 9.0-magnitude earthquake followed by a tsunami pounded northeastern Japan .
NEW : Japan and United States issue different radiation warning zones . Obama emphasizes that the U.S. is determined to do everything possible to support Japan . Kan briefs the president about efforts to contain the nuclear emergency at the Fukushima plant . Kan and Obama talk for about 30 minutes .