Chillan - A column of white smoke and a string of tremors at the Nevados de Chillan have prompted officials to raise the level of alert ahead of a possible eruption at one of the most active volcanoes in Chile.Surrounded by dense forest and rivers, this volcanic complex, which comprises 17 craters, is located some 550 km south of Santiago in the Bio Bio region of the Chilean Andes. Since 2015, the alert level has remained at yellow - the second lowest of four levels - but since December, several eruptions of ash have suggested increased activity, prompting officials to raise the alert by one level, to orange. Unusual lava flows On Friday, teams of police and experts from the state-run National Geology and Mining Service (Sernageomin) deployed to the site -- which reaches an altitude of 3 200 m - saw a huge column of white smoke rising from one of the craters.Sernageomin also detected an unusual flow of lava in the crater which could spill over at any time, and data from 10 monitoring stations, which track the situation by the minute, registered some 4 000 tremors and around 800 explosions. 10 eruptions since 1861Given the likelihood of "a major explosive, eruptive event like we've seen in recent months," the decision was taken to raise the alert level to orange, just below the highest level, said Alvaro Amigo, head of Sernageomin's volcanic surveillance network. Unable to determine exactly when an eruption is likely, experts are constantly monitoring the situation from the volcanic observatory in Temuco, a city in the southern Andes, some 600 kilometers south of Santiago. Between 1861 and 2003, Chillan erupted around 10 times, with varying magnitudes on the Volcanic Explosivity Index.Luxury ski resortsThe last recorded instance was in 1973, although the eruption caused no deaths or major damage.Several ski resorts dot the slopes of Chillan, a popular tourist destination which boasts a clutch of luxury hotels. There are no major urban centers in the area. Chile has about 90 active volcanoes.