The National Weather Service on Saturday warned airplanes to avoid airspace near an erupting Alaska volcano as it spewed ash 30,000 feet above sea level.

Winds were blowing ash from Pavlof Volcano to the west and southwest.

Pavlof began erupting, pushing lava out from a vent near its summit, on Wednesday. On Friday, the ash cloud reached 16,000 feet.

The eruption intensified at 6 a.m. Saturday, sending the ash cloud higher, said Dave Schneider, a geophysicist at Alaska Volcano Observatory.

It’s not clear how long this eruption will last, Schneider said. Pavlof’s eruptions may last for weeks or months with varying levels of intensity, he said.

Pavlof is Alaska’s most active volcano and is located about 625 miles southwest of Anchorage. It’s had more than 40 recorded eruptions, including earlier this year and last year.

It also lies along popular international air routes connecting Europe, North America and Asia.

The eruption has had little effect on people on the ground. The closest community, Cold Bay, is 40 miles away. Observers there reported seeing dark snow on the surface of the volcano Wednesday, indicating an eruption had started.

There have been some small avalanches of hot rock down the north flank of the volcano. These may produce local mud flows to the north, but scientists say they would likely be minor.