Montserrat's Soufriere Hills volcano blasted ash and steam into the air on Thursday, and authorities warned the Caribbean island could see heightened activity in coming days.
The eruption did not affect the northern side of the island, where all of Montserrat's 5,000 residents live, but the Montserrat Volcano Observatory raised the alert status.
"The volcano is extremely active," observatory director Sue Loughlin.
Thurday's blasts sounded like jets flying over the tiny British territory, much of which was buried by an eruption in 1997 that killed 19 people.
The volcano's dome collapsed partially on June 30. It wasn't clear what would result from the latest activity, which included rock falls.
"It's really the uncertainty that causes us to put the alert level up," Loughlin said. "We expect something to happen but we don't know exactly what it might be."
More than half of Montserrat's 12,000 inhabitants have moved away since the volcano sprang to life in 1995.
Access to the deserted southern zone was closed Thursday