US Vice-President Kamala Harris has told Pacific leaders the superpower is going to give the region the "diplomatic attention" it deserves in an "unprecedented" address during a regional meeting at a time of rising geopolitical tensions with China.

"We recognise that in recent years, the Pacific Islands may not have received in diplomatic attention and support that you deserve," said Harris, addressing Pacific ministers virtually today in Suva, with New Zealand Foreign Minister Nanaia Mahuta and Australia's Penny Wong present.

"So today I'm here to tell you directly we are going to change that."

Announcements overnight included an increase to funding for the region, tripling fishing-related assistance to nearly $1 billion over the next decade, and new embassies in Tonga and Kiribati, which just left the forum, sparking concerns its close relations with China could be at play.

Harris spoke at the invitation of Fiji PM and Forum host Frank Bainimarama, who said he "welcomed" the "powerful commitment" saying the US would become a "partner like never before".

In particular Bainimarama made note of the US rejoining the climate change Paris Agreement and pledges to increase ambition in that arena.

As part of initiatives announced overnight, the US is to triple its assistance as part of the US Pacific Tuna Treaty, which grants US boats access to the rich resource, adding up to about NZ$1 billion over the next decade.

Negotiations for the Treaty were ongoing, with original conditions set to expire at the end of this year.

Its new initiatives also include returning peace corps to the region, appointing a US envoy to the forum and establishing a United States Agency for International Development (USAid) Regional Mission for the Pacific in Suva.

Harris also spoke at length of its commitments to climate action and supporting Forum unity.

"You are on the frontline and the world's emissions have had an outsized impact on your nations.

"The evidence of course is that sea levels are rising the coral rates that protect her islands and foster fisheries are bleaching and ocean warming is accelerate.

"This is why as we work with the world to reduce emissions."

She said they would partner on the issue and ensure sustainability of fisheries and marine resources, marine security and disaster relief.

"We also intend to expand our overall economic relationships with you as part of that we will pursue infrastructure projects that are sustainable, high quality, climate-friendly and very important and do not result in insurmountable debt.

"President Biden and I consider our relationship with all of you to be a true partnership and a friendship based on mutual respect, mutual trust and mutual benefit.

"We will also work to empower and strong and united Pacific Islands Forum which will strengthen your voice on the world stage as we continue to work together.

"We will embark on a new chapter in our partnership, a chapter with increased American presence."

Forum Secretary General Henry Puna said the address was "very refreshing, and also very reassuring that the Americans are now fully committed to re-engaging with the Pacific in a meaningful and substantive way".

Asked about how it could be perceived by China, Puna said, "Let's wait and see."

"For us, this is not about geopolitics. Our history, you know, goes back a long way. We've been friends for so long. And it's nice to see that friendship, you know, brought back to life in a meaningful way."

The address and initiatives come hot on the heels of Kiribati breaking away from the Forum. Kiribati has close links to China, switching allegiance from Taiwan in 2019.

It also follows concerns around the Solomon Islands and China security pact, and a push by Beijing to achieve a wider security and economic agreement in the Pacific.

On Tuesday Prime Minister Jacinda Ardern said Harris' address was within the context of fisheries and she did not see the move as anything different from interest in the past.

Ardern said while US and China had been in the Pacific for some time, interest had waxed and waned.

"We have an example of a development partner who's been around the Pacific for decades, but is certainly increasing their activity, and changing the way in which they engage."

She said, on the other hand, the US interest had waned in recent years, and was only now gearing up again.

Prior to the address Dr Anna Powles, a Massey University expert in Pacific security, said it was clearly timed by the US to send a signal to China, which would not have a presence at the leaders' meetings.

US President Joe Biden addressed the forum virtually last year, but Powles said it was unprecedented for a non-forum member to be given time and access through a virtual meeting during leaders' week.

"This is very much about the US seeking to position itself as the partner of choice in the Pacific."

Powles said while some US-aligned members would likely be supportive, the move could upset others as it would take away focus from the forum's unity efforts.

"One thing partners need to learn is the difference between presence and getting in the way."