A fresh cyclone was brewing in the Pacific near the Solomon Islands Thursday as humanitarian groups struggled to get relief supplies to outlying islands in Tonga devastated by Cyclone Ian.

Solomons forecasters said the region's second cyclone of the summer could hit this weekend, a week after Ian slammed into Tonga, killing one person, leaving about 4,000 homeless and destroying crops on outer islands.

Weather forecasts advised Solomon Islanders to keep listening to radio outlets for advice as the strengthening tropical depression in the region "poses threats to lives and properties".

Damage from Cyclone Ian was expected to run to several million dollars in Tonga where the Red Cross had not been able to reach all the affected people in the scattered islands four days after the storm passed.

"We've reached about 95 percent so far," said Red Cross secretary general Sione Taumoefolau who is overseeing the distribution of relief supplies.

The United Nations issued a report Thursday saying more than 50 percent of 1,130 affected buildings on Tonga's central Ha'apai islands, which bore the brunt of Cyclone Ian, were destroyed.

A further 34 percent of the buildings and all but four of the 17 schools "have major damage," according to the report from the UN Office for the Coordination of Humanitarian Affairs.

"(The) immediate challenge is logistics to distribute relief and recovery items to outer islands, as well as within affected areas," it said adding this was being done without a formal call for international assistance.

The Tear Fund New Zealand aid organisation urged the Tongan government to seek international help urgently before the devastation was forgotten.

"The fact is they have a very short but clear window of time to raise support for this, and then it will drop off the radar," Tear chief executive Ian McInnes told Radio New Zealand.

"The needs of the Tongan communities will be eclipsed soon enough in the media by other needs and other issues and the window will have passed.

"My view is that if they want significant international support, they should ask for that now."

Taumoefolau said he expected the government would appeal for help within the next few days when the full cost of the damage was known.

New Zealand and Australia have each pledged US$50,000 so far to recovery efforts.