Chile and Argentina have ordered the evacuation of some 3,000 people living near the Copahue volcano in the south of their shared border.
The authorities in both countries issued a red alert - the highest possible - saying the Chilean volcano could erupt imminently.
The 2,965m (nearly 10,000ft) volcano - which sits in the Andes cordillera - has so far only spewed gas.
Thousands of minor earth tremors have been registered in the area.
The volcano, located between Chile's Bio Bio region and Argentina's Neuquen province, has seen increasing seismic activity in recent weeks but has not erupted.
"This red alert has been issued after monitoring the activity of the volcano and seeing that it has increased seismic activity," Chilean Interior Minister Andres Chadwick said in a news conference on Monday.
"There is a risk that it can start erupting."
Argentina revises alert
According to Chile's Emergency Office, the mandatory evacuation affects some 2,240 people living within a 25km (15 miles) radius of Copahue.
However some people refused to leave their homes, preferring to stay with their farm animals and personal belongings.
"No. I do not want to leave because we have chickens and it isn't easy to leave them and go to a shelter," resident Florinda Lipiman told the news agency Reuters.
In Argentina, the authorities had first declared a "yellow alert," but later revised it to the highest level.
They have now ordered the evacuation of at least 600 people from the town of Caviahue to the neighbouring city of Loncopue.
"We are going to go to the homes and we are going to remove the women, the elderly and children," police officer Javier Urrutia Vergara told Reuters.
Last December, Chile also issued a red alert after Copahue - one of the most active volcanoes in the region - began spewing ash and gas, with smoke raising nearly 1.5km in the sky.
Nearby residents were temporarily evacuated, and planes flying over the southern Andes warned to avoid the area.
Hundreds of flights were cancelled last year due to the eruption of another volcano in southern Chile.
The Puyehue eruption caused huge economic damage not only to property in the area but also to tourism in Bariloche and other resorts.