President Barack Obama sent condolences to the people of Japan on Friday and said the United States stood ready to help its close ally in the wake of a massive earthquake and tsunami.
The U.S. Defense Department was preparing American forces in the Pacific to provide relief after the earthquake, which generated a tsunami that headed across the Pacific Ocean toward Hawaii and the west coast of the U.S. mainland.
Hundreds of people were killed in Japan, and the death toll was expected to rise.
"The friendship and alliance between our two nations is unshakable, and only strengthens our resolve to stand with the people of Japan as they overcome this tragedy," Obama said in a statement.
His chief of staff, Bill Daley, awakened the president to tell him about the earthquake at about 4 a.m. EST (1000 GMT).
The U.S. military effort included at least six U.S. Navy ships, although defense officials had not yet received a request for assistance from the Japanese government, Pentagon spokeswoman Navy Commander Leslie Hullryde said.
"We are positioning our forces so we are ready to respond and provide disaster relief, if requested," she said.
The U.S. State Department said it has moved its embassy operations in Japan from Tokyo to an alternate location as a precaution.
"Our embassy has been in touch with the Japanese government and stands ready to provide any assistance in response to this horrible tragedy," the State Department said in a recorded telephone message in Washington.
There have been no reports of Americans being killed or injured in the quake.
The State Department issued a travel alert strongly urging U.S. citizens to avoid tourism and nonessential travel to Japan.
"Tokyo airports are currently closed; other airports in Japan may be closed or have restricted access. Public transportation, including trains and subways are closed in the Tokyo area, and service has been interrupted in other areas. Many roads have been damaged in the Tokyo area and in northern Japan," the alert said.
"Strong aftershocks are likely for weeks following a strong earthquake such as this one," it added.
The State Department also said it would set up an email address and telephone number to handle inquiries from people with relatives in Japan.
U.S. Vice President Joe Biden, who was on a visit to Moldova on Friday, also sent condolences as he met with Moldovan Prime Minister Vlad Filat in Chisinau.