Asahi reports that according to TEPCO, the operator of the troubled nuclear power plants in Japan, U.S. military personnel were involved in fighting the fire in Reactor Number 4 of the Fukushima Daiichi nuclear power plant. A Reuters item here says TEPCO asked US military forces in Japan for help.
The fire was successfully extinguished.
I can't find details on the reported US military assistance at this time, and have put out queries to military public affairs contacts.