The withdrawal of the remote Pacific island nation of Kiribati from the region's key diplomatic group has overshadowed the Pacific Islands Forum as leaders arrive in Fiji for the first in-person meeting in two years.

Pacific island leaders will discuss how to gather more international support and funding to fight the impact of rising sea levels and climate change during their July 11-14 meeting, as well as China's ambitions for greater security ties across the region.

China's security deal with the Solomon Islands, and a bid by China to sign a broader regional trade and security deal with 10 nations that recognise China, but is opposed by some forum members, was also to be discussed.

Tuvalu Foreign Minister Simon Kofe said he was surprised and saddened by the news of Kiribati's withdrawal and Pacific leaders would need to "look at the concerns raised by Kiribati" when they meet.

Foreign ministers from 16 South Pacific nations had agreed to keep talking to Kiribati, which is seen as being close to Beijing, to "emphasise the strength of the Pacific family", notes from a meeting held by ministers on Friday show.

"Many of us around the table were not fully aware of the extent Kiribati was moving towards withdrawing from the forum," Kofe told Reuters in an interview on Monday in Suva.

"It is disappointing ... It is now up to the leaders to find a way forward," he added.

Kiribati President Taneti Maamau said in a letter his country would withdraw from the Forum because it did not agree with forum secretary-general Henry Puna continuing in the role, nor other terms of a deal brokered weeks ago to solve a rift between Micronesian states and other Forum members. Kiribati also wanted the meeting delayed.

"Kiribati has taken the sovereign decision to withdraw from the Pacific Island Forum with immediate effect," said a letter from Kiribati to the forum secretariat, circulated to several Pacific media outlets on Sunday.

The Kiribati president's office did not respond to a request for comment and forum secretary general Puna has not commented.

A key forum leaders retreat will be held on Thursday. China had proposed a video meeting between the Chinese Communist Party's international office and 10 Pacific islands on the same day.

Kofe said the "geopolitical competition and tug of war that we are seeing right now between China and the U.S." had been discussed in Friday's meeting, and ministers had said "the Pacific really needs to define their future themselves and the terms in which we will engage China and the U.S.".

He said climate change was the major issue, and must be a collective focus.