Former Kiribati president Anote Tong suspects a major agreement is "cooking" between Beijing and Tarawa after the country's decision to quit the Pacific Islands Forum.

Kiribati President Taneti Maamau's "surprise" announcement to abandon its membership from the region's premier policy and political body at the 51st Forum Leaders' meeting this week has heightened concerns the Micronesian nation is moving closer to China.

"I know they are cooking something with China," Tong, who lead the atoll islands from 2003 to 2016, said.

"I think it would have started with the reopening of the Phoenix Island Protected Area."

The Phoenix Islands Protected Area is the largest designated marine protected area in the world, spanning almost 400,000 square kilometres in the South Pacific Ocean, midway between Australia and Hawaii.

Sources have told RNZ Pacific that a possible deal may include exclusive access to Chinese vessels to Phoenix Islands Protected Area.

Tong believed the move by the Maamau government suggested that it hoped to "gain from being isolated from the region" by striking a deal directly with China.

"It's totally unexpected. I did not think it was in our nature, in our character to do something quite so radical like that," he said.

The Kiribati Government is under financial pressure due to the economic impacts of Covid-19 and the current drought faced by the country.

"I know that the government is in a serious problem with the escalating budget which is not sustainable," Tong said.

He said it should not come as a surprise if the government was talking about a deal directly with the Chinese about the Phoenix Islands.

"I have seen expressions in the past in which the president [Maamau] confirmed China was going to assist in the development of Canton Islands ... a former US military base and it was in closer proximity to Hawaii. So, we are very strategically located," he said.

"It is the reason why Kiribati may have withdrawn from the Pacific Island Forum."

China, however, has denied allegations that it has anything to do Kiribati's decision, saying it "does not interfere in the internal affairs of Pacific Islands countries".

Kiribati said it did not feel its concerns over the leadership rift had been listened to following the special meeting hosted by the forum chair Fiji in June, and as a consequence it had no other alternative but to leave.

Federated States of Micronesia president David Panuelo said while it did not know if China was exerting its influence to force Kiribati out of the forum "we hope to find out soon".

Panuelo said there was a lot of work put into the Suva Agreement to achieve a reform package which would see that the forum was "much strengthened."

"Our aim is to open the doors and continue to invite Kiribati because when one member is not on board, it is not quite over yet," he said.

Tong said he "completely disagrees" with Maamau's reasoning.

"I am not so sure that it really provides the justification for the kind of reaction to just withdraw like that.

"There should have been a lot of room to manoeuvre the discussions in Suva. There is so much at stake in losing membership of the forum. I cannot imagine how Kiribati would win by having taken that step."

Tong also raised concerns about the recent visit by China's foreign minister to Tarawa.

"The Chinese foreign minister went through here for a few hours last month and there was a deal signed," he said.

"Nobody knows what that deal is. And so that maybe part of the whole process."