A growing lava dome has been spotted in the summit crater of an Alaska volcano, leading officials to raise the alert level.

The Alaska Volcano Observatory says the observation increases the possibility but doesn't guarantee an eruption at Cleveland Volcano.

Officials say the dome has grown from about 131 feet to 164 feet in diameter since Friday. Weak thermal anomalies also have been detected.

If the dome continues to grow, lava may flow onto the volcano's flanks. Officials say this would not be hazardous to airplanes. Short-lived eruptions could send an ash cloud up to 20,000 feet.

There is no real-time seismic network at the volcano, located 939 miles southwest of Anchorage on an uninhabited island. Officials are not able to track local earthquake activity related to volcanic unrest.