Flight from Chile and Argentina have had to be cancelled after an ash cloud from volcano Calbuco in southern Chile erupted unexpectedly on Wednesday and blew into both countries.
The volcano, which last had a major eruption in 1961, has belched a spectacular plume of ash and smoke around 15 kilometers into the sky near Puerto Varas, some 1,000 kilometers (625 miles) south of Santiago.
Flights were cancelled from the Chilean city of Puerto Montt, and Bariloche in Argentina, popular destinations for domestic and international tourists. Volcanic ash can damage aircraft and makes flying dangerous.
In 2011 the eruption of Chilean volcano Puyehue sent ash high into the atmosphere, leading to the cancellation of flights as far away as Australia.
Volcano Calbuco has paused in its eruption, but remained unstable and could erupt again, Gabriel Orozco, a vulcanologist from the government geology service, told local TV.
"The eruptive column has already cleared away from the volcano and is being taken north by winds," he said.
Most of the Calbuco ash cloud is blowing over the mountains north-east into Argentina, and although ash flow and winds were weak, ash had reached as far north as the region of Maule, less than 300 kilometers from Santiago, said Chilean government meteorologist Arnoldo Zuniga.
The Argentine meteorology service forecast the cloud could reach as far as La Pampa in the center-west of Argentina on Thursday.
Over 4,000 people have been evacuated from the immediate area, authorities said, with the emergency services focusing efforts on the small town of Ensenada, some 15 kilometers from the volcano. TV pictures showed a thick layer of grey ash coating nearby towns.
No one has been reported killed, although a climber who was close to the summit when the eruption occurred is missing.