In a March 14, 2011 photo provided by the Navy Visual News Service, Mineman 2nd Class Cody Stone of Phoenix, Az., assigned to Naval Air Facility Misawa (NAFM), recovers a spool of fisherman netting and returns it to a local civilian during a volunteer cleanup Monday, at the Misawa Fishing Port in Misawa, Japan. (AP Photo/Navy Visual News Service, Mass Communication Specialist 2nd Class Devon Dow)

WASHINGTON - President Barack Obama says he has offered Japan any assistance the United States can provide as it recovers from "multiple disasters."

Obama said Monday he continues to be heartbroken by the images of devastation that have struck the U.S. ally. He pledged during an appearance at a school in Virginia to stand by the people of Japan in the wake of the earthquake and tsunami that delivered a double-blow to the island nation and left thousands dead or missing.

The natural disaster also has triggered a potential nuclear crisis, as explosions there have struck at two power plants.

"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."

United States relief efforts have been affected by the release of radiation from the nuclear plants.

The U.S. Navy says it has moved several U.S. ships away from the troubled Japanese nuclear installation after detecting low-level radiation on 17 helicopter crews positioned there for relief efforts.

Navy Cmdr. Jeff Davis, a spokesman for the U.S. 7th Fleet, said Monday that the Navy is committed to continuing the operation to help the Japanese after last week's earthquake and tsunami. But he says officials had to figure out how to continue safely after airborne radiation was detected Sunday by the carrier USS Ronald Reagan and on a helicopter crew returning to the ship from search and rescue operations.

By moving the ships in the carrier group out of the downwind path of the power plant, Davis says the navy can continue with less risk to Americans participating.

CLOSER TO HOME

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AFTERMATH IN JAPAN

Obama: U.S. will stand by longtime ally Japan

Tide of 1,000 bodies overwhelms quake-hit Japan

U.S. moves ships out of path of Japan radiation