A major 7.0 magnitude earthquake has struck Alaska, causing people to run from buildings and briefly prompting a tsunami alert for coastal areas.


The quake was centred about seven miles (11km) north of Anchorage, the largest city in the state, according to the US Geological Survey (USGS).


A series of aftershocks was recorded following the initial shock, which had a depth of 40.9km, the USGS said.


Around 300,000 people live in Anchorage with 100,000 in the surrounding area.


The earthquake was reported at 08.29 local time (17.29 GMT), the USGS added. One of the more powerful aftershocks that followed minutes later had a magnitude of 5.7.


So far, officials have registered 40 aftershocks: 10 with magnitudes above 4.0 and three with magnitudes above 5.0, the Alaska Earthquake Center reported.


The quake was picked up by seismometers across the country in Boston, thousands of miles away.


A tsunami warning issued for the Cook Inlet and southern Kenai Peninsula regions was later cancelled.


Alaska's Governor Bill Walker has issued a declaration of disaster.


An earthquake of magnitude 7.0 and higher can cause, at minimum, considerable damage in normal structures and can destroy poorly constructed structures, according to the USGS.


Officials are encouraging people to use social media and text messages to contact loved ones as phone lines may be overwhelmed.


"Stay off phone lines if you possibly can, we need phone lines open," said Anchorage Fire Department Chief Jodie Hettrick. "Keep an eye on each other, check on your neighbours - especially if they are elderly or disabled."


There are currently no reports of casualties.