Tongan authorities say fresh volcanic activity at the Home Reef volcano, near the northern islands of Vava'u, is creating a steam cloud two kilometres high.
Government geologist Rennie Vaiomounga says a local pilot first reported the activity on Tuesday afternoon.
Mr Vaiomounga says that report has since been confirmed by a boat in the area.
"We got the report from the Real Tonga airlines pilot about they saw a plume coming out from the area which we roughly estimate the area should be from the Home Reef about 40 nautical miles southwest of Vava'u. It's one of the submarine volcanoes."
Mr Vaiomounga says a team will be sent to monitor the activity once weather conditions have improved.
The Home Reef volcano last had signficant activity in 2006.
One month ago volcanic activity disrupted flights and created land in the Hunga Ha'apai area further south.