One of Alaska's largest and most active volcanoes is restless again, prompting scientists to issue an aviation advisory.

Alaska Volcano Observatory scientists increased the threat level of Mount Veniaminof from yellow to orange Tuesday. That colour designation indicates that sudden explosions could send ash above 6,100 metres and threaten international airplanes.

The observatory's co-ordinating scientist, David Fee, says the 2,500-metre volcano became active Saturday. Fee says small and sparse ash emissions seen rising to the 3,050-metre level prompted the threat level change.

The volcano erupted for several months in 2013. Other recent eruptions occurred in 2005 and between 1993 and 1995.

Veniaminof is 772 kilometres southwest of Anchorage on the Alaska Peninsula.

Fee says Cleveland Volcano in the central Aleutian Islands is Alaska's most active volcano, with the last explosion in May.