The country’s Institute of Territorial Studies said the blasts were due to magma moving inside the volcano.
It urged nearby residents to be prepared for possible evacuation should San Cristobal’s activity increase significantly.
The U.S. Volcanic Ash Advisory Center issued a statement for aviation interests saying that ash was moving westward at or below a height of 5 miles toward the Pacific.
Local media reported that a “giant plume of smoke” was being expelled from the volcano after an initial explosion at 8:15 p.m. local time.
A series of smaller blasts was reported shortly afterward.
Cone-shaped San Cristobal soars to a height of 5,725 feet about 70 miles northwest of Managua, near the Honduran border.
It last erupted in 2006, but a more powerful eruption occurred in 1999.
Photo: Instituto Nicaraguense de Estudios Territoriales