Oakland: The tsunami generated by a massive earthquake in Japan could reach six feet (2 metres) when it hits parts of the northern California coast and force some evacuations, a state emergency agency spokesman said.
In Hawaii, the first signs of a tsunami began to appear, as waves steadily rose over southern beaches on the island of Oahu. The initial waves in Hawaii appeared to have caused no damage.
The massive 8.9 magnitude quake in Japan triggered tsunami warnings for most of the Pacific basin, including northern California and Oregon.
"It is very possible there may be some evacuations here," California Emergency Management Agency spokesman Jordan Scott said by telephone, adding that the far northern California area near the Oregon border was most likely to see big waves.
In Hawaii, some 3,800 miles (6,200 km) from Japan, the main airports on at least three of the major islands -- Maui, Kauai and the Big Island of Hawaii -- were shut down as a precaution, and the U.S. Navy ordered all warships in Pearl Harbor to remain in port to support rescue missions as needed.
Civil defense officials ordered all Hawaiian coastal areas evacuated by 2 a.m. local time, about 90 minutes before the first wave reached the islands at about 8:30 a.m. EST/1330 GMT.
Authorities also ordered evacuations from low-lying areas on the U.S. island territory of Guam in the western Pacific, but the tsunami warning there was lifted several hours later and roads there were reopened.
The Pacific Tsunami Warning Center in Honolulu said the tsunami warning was posted from Mexico down the Pacific coast of South America. The advisory was later extended to a stretch of the U.S. West Coast from Point Conception, California, north through Oregon.
President Barack Obama, a native of Hawaii, was notified of the massive Japanese quake at 4 a.m./0900 GMT and instructed the Federal Emergency Management Agency to be prepared to affected U.S. states and territories, the White House said.
The quake off Japan's northeast coast was the biggest in 140 years and triggered tsunami waves of up to 30 feet (10 metres) that swept across farmland, carrying away homes, crops, vehicles and triggering fires.
On Easter Island, a Chilean territory in the South Pacific, authorities planned to move residents to higher ground hours before a possible tsunami was expected to reach the volcanic isle on Friday afternoon.
Chilean President Sebastian Pinera, whose country was hit by a devastating 8.8 magnitude quake and ensuing tsunamis that killed more than 500 people a year ago, called on Chileans to remain alert, but to continue with their daily routines.