(Reuters) -The Shiveluch volcano in Russia's far eastern Kamchatka peninsula may be gearing up for its first powerful eruption in 15 years, scientists say.
Kamchatka is home to 29 active volcanoes, part of a vast belt of Earth known as the "Ring of Fire" which circles the Pacific Ocean and is prone to eruptions and frequent earthquakes.
Most of the peninsula's volcanoes are surrounded by sparsely populated forest and tundra, so pose little risk to local people, but big eruptions can spew glass, rock and ash into the sky, threatening aircraft.
According to the United States Geological Survey (USGS), these kinds of eruptions typically happen three or four times a year in Kamchatka, requiring air traffic to be rerouted.
Six volcanoes in Russia's northeast are currently showing signs of increased activity, including Eurasia's highest active volcano Klyuchevskaya Sopka, which began erupting last Thursday.
Shiveluch is one of the largest and most active volcanoes in Kamchatka, having erupted at least 60 times in the past 10,000 years.
It has two main parts: Old Shiveluch, which tops 3,283 metres (10,771 ft), and Young Shiveluch - a smaller, 2,800-metre peak protruding from its side.
Young Shiveluch lies within an ancient caldera - a large crater-like basin that likely formed when the older part underwent a catastrophic eruption at least 10,000 years ago.
It is this part that has become extremely active, the Kamchatka Volcanic Eruption Response Team (KVERT) said on Sunday, warning that the volcano's lava dome continues to grow and that stronger "fumarole activity" has been observed.
Domes are mounds that form from accumulating lava, and fumaroles are openings through which hot sulphurous gases emerge.
An eruption may pose a risk to international flights and has been marked with an "orange" threat level, KVERT said, meaning that it is more likely to erupt.
The volcano has been continuously erupting since August 1999, but occasionally undergoes powerful explosive events, including in 2007, when NASA said it ejected a large ash cloud 9,750 metres (32,000 ft) into the sky.
Alexei Ozerov, a director at the Institute of Volcanology and Seismology of the Far East Branch of the Russian Academy of Sciences, said on Sunday the volcano's dome has become very hot.
"The dome glows at night from nearly all sides. Red-hot avalanches with a temperature of 1,000 degrees Celsius (1,832°F) roll down the slopes," he said.
A representative from the institute told Reuters this typically happens before a powerful "paroxysmal eruption", which can happen at any time.
"When the paroxysmal stage of eruption occurs, pyroclastic flows (a mixture of hot gas, ash and rock) will descend from the slopes of Shiveluch at high velocity," they said.
"Ash emissions pose a very great danger to aviation. Tourists should refrain from visiting the volcano area within a radius of 25 kilometres."
At least two earthquakes, including one of magnitude 5.7 at a depth of 21 kilometres, were detected off the east coast of the peninsula on Tuesday.
(Reporting by Caleb Davis, Editing by William Maclean and Tomasz Janowski)