Blast reported at nuclear plant amid worries that quake-hit reactor can no longer cool radioactive substances.
The US has promised to help Japan after a devastating earthquake and tsunami ravaged the island nation.
It is in a special position to do so, as 38,000 US military personnel are already in Japan.
Six naval ships, loaded with helicopters, medical equipments and crew, are sent to aid rescue efforts.
Al Jazeera’s Patty Culhane reports.