All U.S. military and federal government personnel based in Japan are accounted for with no reports of injuries or serious damage to U.S. military facilities there in the wake of the 8.9 earthquake that has rocked Japan.
Japan is one of the United States' closest allies in the region, and the two countries routinely conduct joint military exercises.
The United States maintains several large bases in Japan with 38,000 military personnel, 43,000 dependents and 5,000 Department of Defense civilians.
The main bases are Yokota Air Base outside of Tokyo, the naval bases at Yokosuka in Tokyo Bay and Sasebo in Nagasaki, and the Marine and Air Force bases on the island of Okinawa.
A Defense Department official said there has been no damage to any of them.
Several large civilian airliners were diverted to the runway at Yokota because they could not land at Narita International Airport in Tokyo. Pictures posted on the base's website show at least five planes on the tarmac, including a Delta Boeing 747.
The United States is mobilizing efforts and redirecting its assets in the region to aid the Japanese after the devastating, 8.9 magnitude earthquake and tsunami shook the country today, killing at least 200 people.
"At first, it was absolute panic," U.S. Army Staff Sgt. David Pierson, who is stationed in Japan and was at the Narita Airport in Tokyo when the earthquake struck, told ABC News. "I just saw the lockers in the area start violently shaking back and forth. And then judging by the looks on some people's faces, I figured out what had happened."
Tsunami warnings have been issued throughout the Pacific Rim in the wake of the earthquake. In Hawaii, the Pacific Fleet announced that as a precaution, Ford Island Bridge at Joint Base Pearl Harbor Hickam will be closed beginning at 2:30 a.m. local time. Navy ships at Pearl Harbor in Hawaii have been instructed to stay put and ride out any potential Tsunami wave.
The wave is expected to be 3.5 feet in height at Pearl Harbor.
"There is no intention to move any ships," a Defense Department official said.
President Obama called Japanese Prime Minister Naoto Kan today to discuss the situation.
Kan has requested U.S. assistance, and the Pentagon is now looking into what resources it can provide. The U.S. government began moving assets in the region even before the formal request came.
A USAID Disaster Assistance Response Team is assembling to go to Japan, with their team leader already en route, USAID spokesman Lars Anderson told ABC News. USAID has stood up a Response Management Team in its Washington, D.C. headquarters to oversee logistics of a U.S. response.
A Defense Department official said U.S. Pacific Command is sending some P-3 Orion maritime surveillance aircraft to support the Japanese government by providing aerial reconnaissance over quake-damaged areas.
The Seventh Fleet is "assessing the situation and positioning forces so that they are ready to respond and provide disaster relief if directed," according to a statement.
The amphibious assault ship USS Essex and its Marines aboard will soon depart Malaysia.
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.