The Alaska Volcano Observatory says underground activity is increasing at Cleveland Volcano in the Aleutians Islands.

The observatory recorded an explosion at the volcano at 8:17 a.m. Tuesday, but no ash has been detected. Ash above 20,000 feet would threaten trans-Pacific flights.

The volcano is located on an uninhabited island 940 miles southwest of Anchorage. Its last significant eruption began in February 2001.

In the last 14 years, Cleveland Volcano has occasionally produced small lava flows and explosions with small ash clouds below 20,000 feet.