ANCHORAGE, Alaska – The Alaska Volcano Observatory says lava has flowed into the crater atop an Aleutian Islands volcano.
The circular lava flow was spotted Monday at Mount Cleveland. The flow measured about 260 feet (80 meters) in diameter.
The U.S. Geological Survey says lava flowing over the mountain's active vent increases the possibility of a vent-clearing explosion.
The center raised the aviation warning code to "watch" level.
The USGS says Cleveland Volcano explosions typically produce relatively small volcanic ash clouds that dissipate within hours, but significant ash emissions have occurred.
The last detected explosive activity occurred May 4.
Cleveland Volcano is 940 miles (1,512 kilometers) southwest of Anchorage.
Satellite monitoring also detected a possible steam emission that may be related to lava in the crater.