Published: Mar 14, 2011 7:02 PM EDT
Updated: Mar 14, 2011 4:03 PM EDT

WASHINGTON - President Barack Obama said Monday the U.S. will stand by long-time ally Japan as it recovers from last week's earthquake and tsunami and the nuclear crisis that those twin disasters spawned.

Meanwhile, the Navy reported that several U.S. ships involved in the relief effort had to be moved away from the Fukushima Dai-Ichi nuclear power plant after officials found out that the ships and the 17 helicopter crew members had been exposed to low-levels of radiation. There have been two hydrogen explosions in three days at the plant, a third crisis that developed after the facility's cooling systems failed following Friday's earthquake and tsunami.

Obama said he has offered Japan any assistance the United States can provide as it recovers from the "multiple disasters."

In an education speech at a school in Virginia, the president began his remarks by saying that he continues to be heartbroken by the images of devastation that have struck the U.S. ally. "I know all of you, young and old, have been watching the full magnitude of this tragedy unfold," he told his school audience. He called the people of Japan "some of our closest friends and allies."

Cmdr. Jeff Davis, spokesman for 7th Fleet, said air monitoring equipment on the aircraft carrier USS Ronald Reagan detected that the warship had been exposed to very low levels of contamination. It is presumed that the seven other ships traveling together in the carrier groups also were exposed, but only the Reagan - which has an air monitoring system meant to detect problems with the ship's own nuclear power - picked it up, he said.

Smaller hand-held equipment kept on ships for detecting surface contamination separately determined a low level of exposure for the 17 helicopter crew members who had returned to the carrier after a search and rescue mission over Japan, Davis said.

"Bottom line is, the amount of contamination that they were exposed to was very very low," Davis said from the command ship USS Blue Ridge, which is near the Philippines and headed toward Japan. "It was easily taken care of by washing with soap and water. Once they had discarded their clothing, washed with soap and water and were retested, there was no additional contamination detected.

"The dosage of radiation that they received would have been less than what somebody gets from just normal background radiation over the course of a month from the sun, soil, rocks - all the things around you that give off background radiation," Davis said.

He said officials then moved the ships out of the downwind path of the nuclear plant.

"We are committed to this operation - we're going to do it," Davis said. "We just wanted to make sure that we're doing it in a manner that accounts for the environmental risk."

So far, two U.S. Navy P3-Orion surveillance planes have been mapping debris fields and working as spotters for search and rescue missions, passing on information on the location of victims on to Japanese officials. They have mapped a huge debris field in the water that is one nautical mile wide by 60 miles long and official have recovered some bodies but no survivors.

The U.S. assistance operation will ramp up with the arrival of U.S. Marines, who are expected to use the USS Tortuga amphibious dock ship to pick up some 300 Japanese civil defense workers on the island of Hokaido and ferry them and 90 vehicles Tuesday to the island of Honshu.

The Navy also has agreed to allow use of the deck of the Reagan as a floating platform for refueling Japanese helicopters being flown by the coast guard, police and other civilian agencies in the relief effort.

(Copyright 2011 by The Associated Press. All Rights Reserved.)