Fiji's disaster management office (NDMO) is warning people living on Kadavu Island around Mt Washington, also known as Mt Nabukelevu, to avoid the volcano.
Villagers should refrain from diving and fishing in coastal waters following reports of submarine rumblings and what sounded like explosions, the NDMO said.
People should also avoid the mountain's slopes, the office said, describing the situation as "high-risk".
The NDMO and the Mineral Resources Department were despatching an assessment team to the area today, a spokesperson said.
The situation on Kadavu Island on #MtWashington is currently a priority for the NDMO. We have a team on the ground who are advising communities. Safety of communities remains our PRIORITY. #TeamFiji #FijiNews #Kadavu #MtWashington— Fiji National Disaster Management Office (@FijiNDMO) October 22, 2019
A full media briefing is expected later today.
The warning follows a magnitude 5.1 quake in the area on Sunday, which has been followed by multiple aftershocks.
Nabukelevu's last recorded eruption was in 1660.