ANCHORAGE, Alaska (AP) — A 6.6 magnitude earthquake has rocked buildings in Anchorage and caused lamp posts and trees to sway, prompting people to run out of offices and seek shelter under office desks.

The U.S. Geological Survey says the earthquake Friday morning was centered about 7 miles north of Alaska's largest city.

An Associated Press reporter working in downtown Anchorage saw cracks in a 2-storey building after the quake. It was unclear whether there were injuries.

People went back inside buildings after the earthquake but a smaller aftershock a short time later sent them running back into the streets again.

The National Tsunami Warning Center has issued a tsunami warning for coastal zones of southern Alaska following an earthquake that rocked buildings in downtown Anchorage.

The center said Friday that the warning was in effect for parts of the state's Cook Inlet and the southern Kenai peninsula.

The warning means tsunami waves are expected.