The military assistance operation is known as Operation Tomodachi, or "friendship," the III Marine Expeditionary Force said in a statement. The operation name was chosen by the Japanese.
"We have units from all of our services, with a multitude of capabilities, from medical to communications to civil engineering, poised and ready to support where needed," John Roos, U.S. ambassador to Japan, told reporters Saturday. "Our military is working closely with their Japanese counterparts to support where requested and needed."
In Shiroishi, a town near the area hardest hit by Friday's earthquake, two helicopters from an antisubmarine squadron delivered 1,500 pounds of rice and bread donated by the people of Ebina, a town to the south, the U.S. 7th Fleet said. The fleet is headquartered in Yokosuka, outside Tokyo.
The III Expeditionary Force, based in Okinawa, south of the quake zone, and the 7th Fleet reported no serious damage at their headquarters and no injuries to their personnel.
The destroyers McCampbell and Curtis Wilbur were preparing to move into position off Miyagi Prefecture "to assist Japanese authorities with providing at-sea search-and-rescue and recovery operations." An additional destroyer, Mustin, will depart Yokosuka Sunday.
The Ronald Reagan Carrier Strike Group, which includes a cruiser, a destroyer and a combat support ship, was en route to the east coast of Honshu, expected to arrive Sunday. The aircraft carrier is prepared to serve as a platform for refueling Japanese military and other helicopters involved in rescue and recovery efforts ashore.
The 7th Fleet flagship, the Blue Ridge, took on a Humanitarian Assistance/Disaster Relief kit Friday night in Singapore and departed Saturday morning. It is expected to arrive March 18.
An amphibious assault ship, the Essex, left Mayalsia with elements of the 31st Marine Expeditionary Unit, and is expected in Japan on Wednesday.
U.S. Forces Japan, based at Yokota Air Base near Tokyo, is coordinating humanitarian assistance, the military said. The air base is being used as an alternate for flights that cannot land at Tokyo's Narita Airport.
The U.S. Agency for International Development dispatched two urban search-and-rescue teams, one from Fairfax County, Va., and the second from Los Angeles County. Each has about 72 personnel, dogs and 75 tons of rescue equipment.
"We are working with the government of Japan to provide any assistance needed in the rescue effort as quickly as possible," USAID chief Rajiv Shah said in a statement.
The Virginia team left Washington on Saturday and was stopping in Los Angeles to pick up the second team, officials said.
Both teams are set to arrive Monday morning in Misawa, Japan, where they will "immediately begin the search for live victims" alongside Japanese and other international teams.
A USAID Disaster Assistance Response team is already in Japan, the agency said.
The U.S response is not limited to the federal government.
U.S. non-governmental aid organizations will be pitching in, said Samuel A. Worthington, president of InterAction, a coalition of the groups. "Our community stands ready to help as needed," he said. "Our members have quickly mobilized efforts and are communicating with directly civil society groups in Japan."
California Gov. Jerry Brown directed the state's Emergency Management Agency to make resources available to the Japanese government.