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.