The United Nations and its agencies are readying relief efforts for the remote South Pacific island nation of Tonga following the eruption of the Hunga Tonga-Hunga Ha'apai volcano, a UN spokesman said on Monday.

"Our humanitarian colleagues and the government report significant infrastructural damage around Tongatapu (the main island) and no contact from the Ha'apai group of islands," said Stephane Dujarric, the chief spokesman for UN Secretary-General Antonio Guterres.

"We are particularly concerned about two small islands, Mango and Fonoi, following surveillance flights confirming substantial property damage."

Australia and New Zealand carried out the flights over the weekend.

The first known death in Tonga itself was confirmed: that of a British woman swept away by the tsunami. She was identified as Angela Glover, 50, who lived in the Tonga capital with her husband James.

Dujarric said the World Food Program is mapping efforts to deploy relief supplies and staff. The agency is sending its emergency communications cluster to help restore communications lines in Tonga.

The spokesman said the UN Children's Fund awaits government confirmation before transporting prepositioned emergency supplies from Fiji and Brisbane, Australia. The supplies include essential water, sanitation and hygiene kits, water containers and buckets and water-field test kits, tarps, recreational kits for children and tents.

The UN Office for the Coordination of Humanitarian Affairs (OCHA) said the tsunami that occurred in the aftermath of the massive eruption has damaged the western beaches of Tongatapu, with several resorts and houses destroyed or badly damaged.

Other UN agencies working on relief include the UN Population Fund, the Food and Agriculture Organization, the International Organization for Migration and the World Health Organization, the office said. The agencies work closely with relevant authorities in Tonga, humanitarian partners and donor countries.

The Red Cross offered assistance and the Secretariat of the Pacific Community is providing technical support on the impact of the eruption and ashfall, the office said. The Secretariat of the Pacific Community is an international development organization comprising 22 Pacific island countries and territories.

Tonga has declared a state of emergency, New Zealand's media reported, citing Peter Lund, New Zealand's acting high commissioner in Tonga.

Tonga's National Emergency Management Office is leading assessments and government response. The Tonga Red Cross Society and OCHA provide communication support.

The humanitarian office said there was no serious impact on Fiji, Samoa, Vanuatu and the Solomon Islands besides limited flooding.

(With input from agencies)