Vanuatu's meteorological service says tropical cyclone Ula has been upgraded to a category four, and is now forecast to pass close to the islands of Aneityum and Futuna.
The cyclone is moving in a west-southwest direction at 10 kilometres an hour, and is currently about 225 kilometres east-northeast of Aneityum, and 270 kilometres east of Tanna.
Ula has winds of 165 kilometres an hour close to its centre.
The service says the southern province of Tafea can expect damaging gale-force winds, torrential rain, and very rough to phenomenal seas.
The province was one of the worst affected by the category five Cyclone Pam, which struck in March last year, and has been struggling with drought caused by this year's El Niño system.
Last weekend, Ula caused minor damage in northern Tonga and Fiji's Lau group as a category two system, but it has strengthened again in the past 48 hours.
To the southwest, New Caledonia's government has issued a cyclone pre-alert for the entire territory.