By Rosalba O'Brien and Anthony Esposito
SANTIAGO (Reuters) - An ash cloud from the Calbuco volcano in southern Chile that erupted unexpectedly on Wednesday was blowing into Chile and Argentina on Thursday, forcing the cancellation of flights from nearby cities in both countries and blanketing residents in ash.
The volcano, which last had a major eruption in 1961, has belched a spectacular plume of ash and smoke around 15 kilometers (9 miles) into the sky near the popular tourist town of Puerto Varas, some 1,000 kilometers (625 miles) south of Santiago.
Calbuco erupted twice over the last 24 hours and has now paused, but it remains unstable and could erupt again, experts said.
Winds have already carried part of the ash cloud to the city of Chillan, some 400 kilometers south of Santiago, and could reach the capital by the end of the day, said government meteorologist Arnoldo Zuniga.
Most of the ash, however, was headed over the Andes mountains into neighboring Argentina, and weather experts said it could reach as far as Buenos Aires province, some 1,500 kilometers to the east.
"At the moment we don't forecast the ashes will reach Buenos Aires city, depending on whether or not there is a third eruption," said Ignacio Lopez, a meteorologist at Argentina's national meteorology center.
Volcanic ash can damage aircraft and makes flying dangerous. In 2011, the eruption of Chile's Puyehue volcano sent ash high into the atmosphere, leading to flight cancellations as far away as Australia.
"So far, the eruption has been slightly less serious (than in 2011) and the meteorological conditions have been good, meaning the ashes are remaining suspended in the air," said Lopez.
International airlines that fly into Santiago contacted by Reuters said they were monitoring the situation but flights were unaffected.
In Argentina, Bariloche and Neuquen airports were shut, with Bariloche - which TV pictures showed covered in gray ash - due to be closed Friday too. Flights by LATAM Airlines' local arm LAN Argentina and state-owned Aerolineas Argentinas were affected.
LAN Chile had canceled over 20 domestic flights since the eruption but said on Thursday afternoon that owing to an improvement in conditions as the ash dissipated, it was resuming a normal schedule.
Nearly 4,500 people have been evacuated from the immediate area, authorities said, with the emergency services focusing on the small town of Ensenada, some 15 kilometers from the volcano.
No one has been reported killed.
(Additional reporting by Antonio de la Jara in Santiago and Sarah Marsh in Buenos Aires. Editing by Meredith Mazzilli, Andre Grenon and Andrew Hay)