Rumbling at Villarrica, one of South America's most active volcanoes, has been worrying residents and prompted authorities to declare an orange alert this week.

Authorities in the south of Chile are rehearsing evacuation plans amid concern that the volcano could potentially erupt within "days or weeks."

Villarrica, near the popular tourist resort of Pucon around 750km south of the capital Santiago, was upgraded to an orange alert after it began rumbling and hurling lava into the air late on Tuesday.

The orange alert by the National Geology and Mining Service (SERNAGEOMIN) implies a "significant" increase in activity.

Álvaro Amigo, head of the national network of volcano vigilance for SERNAGEOMIN, said Villarica could erupt within "days or weeks."

"What we have observed in recent days is a significant increase in its activity," he said on Tuesday when the alert was upgraded. "A probable scenario is that this results in a major eruption."

Chile has the second-largest and most active volcanic chain in the world, behind Indonesia.

Villarrica has a 200 metre-wide open crater and last erupted in 2015, sparking a mass evacuation but ultimately no injuries.

A second volcanic complex, the Nevados de Chillan, some 230 miles north, has also been placed on orange alert after geologists reported an uptick in activity. This one might take "months" to erupt, according to SERNAGEOMIN's Amigo.

Residents and visitors around Pucon have been urged by the authorities not to approach within two kilometres of the volcanoes' peaks. The dangers include potentially deadly mud, rock and lava flows, SERNAGEOMIN said.

Managers of the town's ski centre on the slope of Villarrica, which attracts thousands of visitors each year, met with government, army and police officials on Tuesday to revise evacuation plans.

Pucon's mayor, Carlos Barra, told El Mercurio newspaper that since late Tuesday, volcanic activity had virtually ceased and an estimated 50,000 tourists headed to the area for Chile's annual independence celebration holidays should not be alarmed.

"This is a city prepared for this type of event, we have everything signposted, meeting points, good roads, they can come with confidence," he said.