ANCHORAGE, Alaska (Reuters) - A remote Aleutian volcano that has been restless for the past year rumbled to life on Tuesday, shooting a thin cloud of ash several miles into the sky, which could pose a slight hazard to aircraft, Alaska scientists said.

Cleveland Volcano, a 5,676-foot (1,730 meter) peak on an uninhabited island 940 miles southwest of Anchorage, had an explosive eruption at about 2:05 p.m. local time, the Alaska Volcano Observatory reported.

A pilot flying in the area estimated that the ash cloud rose to 35,000 feet above sea level, reported the observatory, which is a joint federal-state organization that monitors Alaska's numerous active volcanoes.

However, satellite imagery shows only a weak ash signal, suggesting a thin cloud that dissipated quickly, said Stephanie Prejean, a U.S. Geological Survey seismologist at the observatory in Anchorage

"It was just one explosion, which was very typical of the thing Cleveland has been doing in the last year," Prejean said. It is possible that the cloud rose to less than 35,000 feet, as the height was just one pilot's estimate, she said.

Pilots have been advised of potential risks from Cleveland, which might explode again, Prejean said. "It could do that any time," she said.

Cleveland has been in an off-and-on eruptive phase since last summer. The volcano has at times oozed lava out of its summit crater, punctuated by occasional small explosions, none of which had created clouds reaching above 20,000 feet until Tuesday.

Although Cleveland is one of the most active of Alaska's scores of volcanoes, there is no on-site seismic monitoring equipment. The Alaska Volcano Observatory must rely on satellite imagery, records of lightning strikes, witness reports and other evidence to determine if an eruption has taken place, Prejean said.

That is in contrast to volcanoes closer to Anchorage, where the observatory has set up seismic-monitoring networks that allow scientists to see signals before eruptions happen, she said.

Before Tuesday, there had been an apparent slowdown in activity at Cleveland. The Alaska Volcano Observatory three weeks ago lowered the alert level for the volcano to a "yellow" advisory from the more urgent "orange" watch status.

"Obviously, it has some oomph left in it," said Tina Neal, an Alaska Volcano Observatory geologist.

Meanwhile, two moderate earthquakes rattled the western Aleutians on Tuesday.

The first, registered at magnitude 6.0, hit Tuesday morning about 90 miles northwest of Attu, the westernmost island in Alaska's Aleutian chain, according to the West Coast and Alaska Tsunami Warning Center. The second, at about noon, measured at magnitude 5.7 and was centered about 65 miles northwest of Attu, the center said.

There were no reports of damage in the largely uninhabited region, and no tsunami warning was issued, said David Hale, a senior watch stander at the center.