An earthquake of magnitude 7.2 has struck the Pacific Ocean 172km east of Atka, Alaska, at a depth of about 40km, the US Geologic Survey (USGS) has said.

The Alaska Earthquake Information Centre says the Thursday evening quake was felt through the central Aleutians and as far east as Dutch Harbor and Unalaska.

There were no immediate reports of damage or injuries.

A second earthquake of the same magnitude hit in the same vicinity and at the same depth a half-minute later.

The West Coast and Alaska Tsunami Warning Centre posted a tsunami warning for certain coastal areas of Alaska, but canceled the warning about an hour after the quake.

The initial warning covered an area from 128km northeast of Dutch Harbour to about 200km west of Adak.

In Dutch Harbour, longshoreman Jim Paulin said warning sirens caused hundreds of people to begin climbing up a nearby hill.

The centre monitors tsunami risk for the west coast of North America, from the Mexican border to Alaska.