Southern Chile's Calbuco volcano erupted for the first time in nearly half a century, spewing a giant funnel of ash 10 kilometers (six miles) into the sky and prompting authorities to declare a state of emergency.

Officials ordered an evacuation for a 20-kilometer radius around the volcano and the interior ministry rushed in the army to temporarily take control of the province of Llanquihue and the town of Puerto Octay.

Emergency measures were also in place in neighboring Argentina, where the picturesque city of Bariloche -- about 100 kilometers from the volcano -- said it was anticipating the arrival of ash clouds within hours and warned people to stay at home.

Puerto Montt, over the border from Bariloche and the largest Chilean city in the area, was already blanketed in a cloud of ash.

"People are very, very frightened," said Gervoy Paredes, mayor of Puerto Montt.

"The situation is pretty complicated," he added, saying reports indicated the Blanco river was flooding because of ice melt triggered by the eruption.

Airlines canceled flights as the towering, mushroom-shaped ash cloud rose from Calbuco's snowy peak.

Schools also suspended classes in the region.

Television images showed large traffic jams and long lines at gas stations in Puerto Montt, where a red alert was declared, along with nearby Puerto Varas.

"I would like to call on the population to remain calm and stay informed," said Interior Minister Rodrigo Penailillo.

"The police have given orders and begun to evacuate."

The initial evacuation involved about 270 families, but could be expanded, officials said.

Police called on residents to keep roads as clear as possible to aid the evacuation.

Calbuco had been inactive for 43 years.

It was a "rather explosive eruption," said Gabriel Orozco, a volcanologist at the National Geology and Mines Service.

He said the 10-kilometer-high ash cloud risked collapsing, and warned: "River beds are very dangerous at the moment" because of the risk of ice and snow melting and causing floods.

The 2,000-meter (6,500-foot) volcano is located in the Los Lagos region, some 1,400 kilometers south of the capital Santiago.

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

That eruption caused authorities to evacuate more than 3,500 people.

No lava was seen in the latest eruption, which began around 6:00 pm (2100 GMT).

But the ash cloud, which could be seen from 20 kilometers away, loomed over the crater, turning hues of pink and yellow as the sun set over the area.

Chile has about 90 active volcanoes, of which Calbuco is considered one of the most dangerous.