Costa Rican authorities have raised the alert level for the Turrialba volcano, after it started spewing smoke and ash into the atmosphere.
The country's National Seismology Network raised the level to yellow, indicating an eruption is imminent.
"The hydrothermal part of the volcano is interacting and giving off decompressions or small explosions in the peak of the southwest crater and this is causing rocks to be thrown and gases," said seismologist Carlos Ramirez.
"The volcano has presented certain events which has led to suspicions of an eruption, such as the movement of fluids inside the volcano," added another seismologist, Lepolt Linkemer.
The last registered eruption from the volcano, which is situated east of the capital San Jose, was in January 2010, when two villages had to be evacuated.