SANTIAGO/BUENOS AIRES (REUTERS) - Chilean and Argentine authorities on Monday declared a red alert and ordered the mandatory evacuation of a 25km radius around the active Copahue volcano, which straddles the border between the two Andean nations.

The volcano - located some 500km south of capital Santiago, between Chile's Bio Bio region and Argentina's Neuquen province - has seen increasing seismic activity in recent weeks but has not erupted, Chilean authorities said.

"This doesn't necessarily mean the volcano will start erupting. But according to the Sernageomin (National Geological and Mining Service), the volcano is now in a process that could culminate in an eruption, for that reason we've issued a red alert and the evacuation," Chilean Interior Minister Andres Chadwick told a nationally televised news conference.

Authorities estimated that some 2,240 people will be evacuated in Chile.

In Argentina's Neuquen province, authorities also declared a "red alert", and ordered the evacuation of some 900 people in tourist-haven Caviahue-Copahue. The Argentine municipality had previously ordered the cancellation of school classes.

Close to the Chilean side of the volcano, in Bio Bio region, power generator Endesa Chile operates the Ralco and Pangue hydroelectric dams, which have not been affected by the evacuation order.

Endesa Chile is monitoring the situation, a company source told Reuters. Water levels at the dams are at technical lows, which would avoid the possible need to open the floodgates, and the dams' walls are designed to withstand earthquakes, the source said.

In mid-2011, ash from a volcano in Chile's Puyehue-Cordon Caulle chain that erupted after decades of lying dormant forced the sporadic cancellation of hundreds of flights, especially in neighbouring Argentina and Uruguay.