Tong said global warming had left small island nations fighting for their future and there was no room for compromise on the issue at this week's PIF meeting in Papua New Guinea.
The 16-nation grouping consists mainly of small island nations including Kiribati, together with Australia and New Zealand, and the two developed nations have been accused of dragging their feet on climate change.
Tong said the island nations wanted to send an unequivocal message ahead of climate talks in Paris later this year that average global warming should be limited to 1.5 degrees Celsius over pre-Industrial Revolution levels.
"It will be incumbent upon them... How relevant would their presence be?" he told reporters.
The Kiribati leader, who last year finalised the purchase of a block of land in Fiji in case rising tides force his people to relocate, said compromise would be "a betrayal of trust".
"We cannot negotiate this, we cannot be bought by aid on this one," he said.
Fiji Prime Minister Voreqe Bainimarama has refused to attend the PIF while Australia and New Zealand remain full members, saying the wealthy nations dominate the body.
He said Australia and New Zealand should be relegated from full PIF members to "development partners", which would give them observer status on the sidelines like the EU, China and India.
The PIF in recent years has taken a leading role in raising global awareness about climate change, with member nations bearing the brunt of its impact even though their carbon emissions are minute.
Island leaders have described the UN-brokered COP21 talks in Paris in December as the last chance for the world to reach a binding agreement that can secure their future.
