A Defense Department official said the aircraft carrier USS Ronald Reagan is being moved to Japan in case needed for helicopter support. The carrier had just arrived in the region to participate in an annual exercise with South Korea.
The State Department is sending an official to Yokota outside Tokyo, where the United States has an Air Force base, to coordinate aid relief. He said there are about 100 helicopters there that can be used.
"The United States stands ready to help the Japanese people in this time of great trial," Obama said in a statement. "We will continue to closely monitor tsunamis around Japan and the Pacific going forward and we are asking all our citizens in the affected region to listen to their state and local officials as I have instructed FEMA to be ready to assist Hawaii and the rest of the U.S. states and territories that could be affected."
The president urged those in the path of the tsunami to evacuate if necessary.
"Our immediate priority is the safety of the people and communities in the affected areas," Obama said. "We remind everyone who lives in the region to monitor their local news for instructions from their state and local officials and if told to evacuate -- evacuate."
The U.S. Navy has a large presence in Japan, which is home to U.S. Seventh Fleet Headquarters and the aircraft carrier USS George Washington. The carrier was pierside at its home port of Yokosuka when the earthquake struck.
Sailors aboard the ship at the time of the quake told Stars and Stripes that they felt the quake begin with a gentle rocking that soon felt as though they were at sea.
State Department spokesman P.J. Crowley said that U.S. Ambassador to Japan John Roos has moved the U.S. Embassy's command center "to an alternate location as a precaution given the many aftershocks in Tokyo."
On a visit to Moldova, Vice President Joe Biden said the United States "stands ready to do anything we can to help our Japanese friends as they deal with the aftermath of this tragedy."
House Minority Leader Nancy Pelosi also offered her condolences and urged her own constituents in California to heed tsunami warnings and instructions from emergency personnel.
"The images of destruction and devastation touch the hearts of families around the world, and members of Congress and all Americans stand ready to offer our aid and support to Japan in its hour of need," Pelosi said in a statement.
The devastating earthquake and tsunami have elevated concerns about the security of nuclear facilities. Despite the fact they are tying to present a calm front, a U.S. official said, the Japanese are "very nervous" about their nuclear plants.
ABC News' Huma Khan, Kirit Radia and Martha Raddatz contributed to this report.