The South Pacific nation of Tonga was lashed by a powerful cyclone on Saturday, with damage reported in the heavily populated northern islands.

A state of emergency was declared for Vava'u and Ha'apai islands on Saturday morning as category-five storm Cyclone Ian brought heavy rain and strong winds forecast to gust at up to 287 kilometers (178 miles) per hour, the Matangi Tonga news website said.

The storm was later downgraded from the top of the scale of destructive cyclones to category four, with gusts of up to 250 kph (155 mph).

The main island of Tongatapu appears to have avoided the worst of the storm.

"We have received a damage report from the island of Hunga and our team are still out trying to get a feel, but there doesn't appear to have been much damage in Vava'u at the moment," Tonga's Director of Emergencies Leveni Aho told the website.

Aho advised against travel between Tongan islands during the storm and urged residents of outer islands to stay put.