A powerful magnitude-6.8 earthquake shook Alaska's remote Aleutian Islands early Friday, prompting a tsunami alert that was subsequently canceled.

The United States Geological Survey detected the temblor at 1055 GMT and said it was located at a depth of 35.5 kilometers (22 miles) in the Pacific Ocean.

The earthquake, which was downgraded from an initial 7.1-magnitude, affected a remote part of the Aleutian Island chain known as the Fox Islands, the USGS said.

There were no initial reports of injuries or damage.

The epicenter of the quake was located 45 kilometers (28 miles) southwest of Amukta Island and 1,658 kilometers west-southwest of Anchorage, the state's largest city.

Shortly after the earthquake, authorities issued a tsunami alert from the island of Unimak, the largest in the Aleutian archipelago, to the remote and sparsely populated island of Amchitka.

Authorities said in issuing the alert they did not expect a destructive tsunami in the Pacific region or Hawaii, although they said they were carefully monitoring a possible threat to the northwest coast of the mainland United States.