Southern Chile's Calbuco volcano erupted again Thursday, releasing a large column of smoke in a towering arc, just over a week after it spectacularly roared to life following half a century of inactivity.

"As predicted, the third eruptive pulse at the Calbuco volcano has arrived. Red alert," the National Geology and Mines Service wrote on its official Twitter account, with experts warning that the volcano may not be done just yet.

A large plume of dark gray smoke and ash rose from the crater, prompting authorities to once again evacuate a 20-kilometer (12-mile) radius around the volcano, where workers and residents had been granted limited access to begin the clean-up effort.

"All of a sudden they ordered us to evacuate again so we left in our cars. But it was calmer than last time," said Horacio Camano, a resident of La Ensenada, a town of 1,500 people at the foot of the volcano.

TV images showed thousands of people rushing to schools to pick up their children or lining up at gas stations to fill up their cars in the cities closest to the volcano, Puerto Montt and Puerto Varas.

Officials said the latest eruption was less powerful than two previous ones last Wednesday and Thursday, when Calbuco surprised residents of the Los Lagos region by rumbling to life after 54 years of inactivity, prompting authorities to evacuate more than 6,000 people and declare a state of emergency.

But the volcano remains unstable, warned the head of the National Geology and Mines Service, Rodrigo Alvarez.

"This third eruption does not mean the eruptive situation of the Calbuco volcano is over," he said.

Last week the volcano belched a 15-kilometer column of ash into the air before spewing bursts of bright orange and red lava into the night sky, criss-crossed by lightning bolts generated by the static electricity built up in the magma.

The twin eruptions ejected ash that blanketed the region and disrupted flights across a broad swath of South America.

The latest eruption was "considerably smaller," said Deputy Interior Minister Mahmud Aley.

"The ash column does not rise higher than four kilometers," he said.

"We shouldn't see any major complications, apart from those that residents will face to the south of the volcano," where the ash cloud was drifting, he said.

Across the border in Argentina, the resort town of Bariloche declared an alert, and several flights were cancelled at the local airport.

- Covered in ash -

Prior to last week, the 2,000-meter volcano had last erupted in 1961 and showed light activity in 1972.

It is located about 1,300 kilometers south of Santiago, in a region popular with tourists for its scenic mountain landscapes dotted with volcanoes and lakes with black-sand beaches.

There have been no reports of casualties from any of the eruptions.

It is the second volcano to erupt in Chile since March 3, when the Villarrica volcano emitted a brief but fiery burst of ash and lava.

Chile has about 90 active volcanoes.

The long, thin country has been hit by a series of natural disasters in recent months, from flooding in its usually arid north, home to the world's driest desert, to wildfires in its drought-hit southern forests.

Last week's eruptions will cost the Los Lagos region an estimated $600 million, said the head of the Puerto Varas Chamber of Commerce.

Besides its thriving tourism economy, the region is the heart of Chile's salmon industry, the world's second-largest, and has around 30 percent of the country's cattle.

Many salmon farms had their water supplies contaminated by the volcanic ash, which also covered large amounts of farmland, forcing the evacuation of hundreds of cattle and thousands of sheep.