The authorities in Ecuador have begun evacuating people from the slopes of the Tungurahua volcano after it started spewing ash.
Scientists say fast-moving currents of extremely hot gas and rock could be seen flowing from the volcano's crater.
Tungurahua, some 135km (85 miles) southeast of the capital, Quito, has been in an active state since 1999.
But experts say there has been a rapid increase in its seismic activity since Saturday morning.
Scientists with the Ecuadorean Institute for Geophysics say the number of explosions has increased. They say the ash cloud has reached 2km (1.2 miles) in height.
People living on the slopes reported the ground and buildings shaking, and a rumbling sound coming from the volcano.
Hot gases and rocks started flowing down the western side of the mountain at mid-morning and ash has been raining down on the villages of Pondoa and Patate.
The authorities have put the region immediately around the volcano under red alert and the security forces are preventing people from entering the area.
The tourist town of Banos is one of those being evacuated.
In 1999, its 15,000 inhabitants were forced to evacuate when the Tungurahua had its last major eruption. Residents were not able to return to their homes for a year.