(CNN)Tsunami warnings have been issued for the islands of Tonga and Samoa, after an underwater volcano in the South Pacific exploded in a violent eruption on Saturday, sending a cloud of ash and gas steam into the air.

The eruption of the Hunga-Tonga-Hunga-Ha'apai volcano has been ongoing since Friday, according to United States Pacific Tsunami Warning Center.

Tonga Meteorological Services have also issued advisories for heavy rain, flash flooding and strong winds in lands and coastal waters.

Nearby, New Zealand's official weather service, said its weather stations across the country had observed "a pressure surge" on Saturday evening from the eruption.

The volcano had been active from December 20, but was declared dormant on January 11, according to CNN affiliate RNZ.