(CNN) -- Two Alaskan volcanoes that began minor eruptions weeks ago are showing their strongest seismic activity yet, emitting small amounts of lava and ash along with smoke plumes, the Alaska Volcano Observatory said Tuesday.
The observatory warned of more vigorous activity with the Pavlof and Veniaminof volcanoes, both on the Alaskan Peninsula -- though that will likely just mean more ash.
Pavlof, a snow-covered, cone-shaped mountain, has been erupting since early May. While its activity since then has waxed and waned, seismic activity increased since Tuesday morning and the volcano has started continuously shaking, said David Schneider, a geophysicist with the U.S. Geological Survey.
Some lava is flowing, and the volcano is spewing a cloud of steam and ash that has risen as high as 28,000 feet, said Schneider, at the observatory in Anchorage.
"It's a pretty good size, but not high enough to affect overflying aircraft between America and Asia," he said. "It's more of a problem for local aviation."
Pavlof is near the town of Cold Bay, a regional transportation hub whose long World War II-era runway serves flights to area villages, Schneider said. The ash has at times affected those local flights.
Veniaminof, about 60 miles from Pavlof, started showing activity in early June and began erupting June 13, Schneider said. It has also produced a lava flow and an ash plume, though not as high as Pavlof -- only about 8,200 feet.
Veniaminof is a broad mountain topped with a large, flat ice field, and in the middle is a small cone that produces the lava and ash.
The eruptions of both volcanoes compare to those in the past, where activity lasted for weeks or months before dying down again. The main hazard now, Schneider said, is ash fall, which is more of a nuisance than anything else.
"So far there's been trace amounts of ash, so it's just a fine coating," he said. "Our best estimate of what is going to happen is similar to what's happened in the past -- this level of activity and some level of ash."