Tonga’s main island sustained significant damage from a volcanic eruption and tsunami, New Zealand’s leader said, promising rapid aid for a Pacific archipelago that remains largely cut off from the world a day after powerful waves crashed onto its shores.

An undersea volcano erupted on Saturday, generating a gigantic ash cloud and triggering a tsunami that was centered on Tonga, and prompted advisories to stay away from beaches in New Zealand, Australia, Japan and parts of the U.S.’s West Coast.