ANCHORAGE, Alaska, July 21 (UPI) -- Officials issued a red alert for pilots following the strengthening of an ongoing eruption of the Aleutian Islands' Okmok volcano.

The volcano, located 850 miles from Anchorage, Alaska, calmed down Sunday after spewing water, debris and gas some 25,000 feet into the air Saturday, the Alaska Volcano Observatory Center reported.

Melissa Pfeffer, a geochemist at the observatory, told the Anchorage Daily News that clouds were preventing scientists from getting a good look at the volcano.

The 3,500-foot volcano began erupting July 12. When it became more active on Saturday, the observatory issued a red alert to warn pilots in the area to watch out for ash.

Sunday's ash plume rose 12,000 feet.

Okmok's last previous eruption was in 1997. In 1817, an Okmok eruption destroyed an Aleut village.