Alaska and parts of western Canada braced for a possible tsunami on Tuesday after a magnitude-7.9 earthquake struck the Gulf of Alaska, sparking evacuations in coastal Alaska and a tsunami warning for California that was later lifted.

In Alaska, people packed into high schools and other evacuation centers after the quake hit shortly after midnight local time (0900 GMT).

Officials had warned residents as far south as San Francisco to be ready to evacuate coastal areas but later lifted tsunami watches for California, Oregon and Washington states as well as coastal British Colombia in Canada.

In Alaska, where a tsunami advisory remained in place as of 3:12 am local time (1212 GMT).

Residents gathered in shelters on Kodiak Island, the closest land point to the quake, around 160 miles (250 km) southeast of Chiniak, Alaska, at a depth of 25 km - considered shallow but with broader damage - according to the US Geological Survey.

There were no immediate reports of injuries or damage from the quake, which was initially measured at magnitude 8.2.

"People are fine," said Neil Hecht, assistant principal of Kodiak High School, which was sheltering a few hundred people. "Spirits are high. Everyone is doing well here."

Long lines of traffic formed in coastal communities including Homer and Seward, Alaska, residents warned on social media.

In Homer, a few hundred cars were packed into its high school parking lot. Shawn Biessel, a 32-year-old park ranger, and his mother were in the lot, a few hundred feet above sea level.

"It was a really obvious, pretty strong, long quake. A good rumbler," Biessel said in a phone interview. "It went on for a solid minute and after a while we thought we should get outside."

Police drove through Biessel's neighborhood with flashing lights to alert residents to evacuate, Biessel said.

"Please heed local warnings to move inland or to higher ground," Alaska Governor Bill Walker said in a statement.

San Francisco's Department of Emergency Management briefly warned residents within three blocks of the Pacific Ocean or five blocks of San Francisco Bay to prepare to evacuate. That warning was lifted when the tsunami watch was lifted.

An initial tsunami watch for Hawaii was cancelled.

Japan's meteorological agency said it was monitoring the situation but did not issue a tsunami alert.