The leaders of Papua New Guinea, Vanuatu and Solomon Islands will not attend this week's annual Pacific Islands Forum, posing a challenge for regional unity amid competition for influence between China and the United States.
The Melanesian nations, which have been courted by China and the US for security and economic ties in the strategic South Pacific, will instead send ministers to the 18-member bloc meeting opening in Rarotonga, Cook Islands, on Monday.
Papua New Guinea will be represented by Deputy Prime Minister John Rosso, officials of the bloc's most populous nation told Reuters, without explaining Prime Minister James Marape's absence.
Vanuatu Prime Minister Charlot Salwai will send Climate Change Minister Ralph Regenvanu.
That nation's focus at the four-day meeting will be "strengthening regionalism" though that would be challenging with key nations not sending leaders, Regenvanu said, without elaborating on why Salwai will not attend.
Vanuatu, whose largest external creditor is China, has experienced a period of political instability, with Salwai the third prime minister since August.
Solomon Islands' government said Foreign Minister Jeremiah Manele would lead its delegation.
Prime Minister Manasseh Sogavare was "tied down with duties as Minister for Pacific Games" and parliament, his office said. Solomon Islands hosts the games this month.
The Pacific Islands Forum is comprised of nations from three sub-regions - Melanesia, Micronesia and Polynesia - as well as Australia and New Zealand.
It has said the region's greatest challenge is rising sea levels and worsening storms caused by climate change.
The United States last month pledged to build more undersea telecommunications cables and wharves in the region, as it seeks to compete with China's influence after that country's decade of infrastructure lending.
China struck a security pact with the Solomon Islands last year and unsuccessfully sought to sign 10 Forum members to a trade and security deal, prompting concern from Australia, the region's biggest aid donor.
Australian Prime Minister Anthony Albanese, who will meet Chinese President Xi Jinping in Beijing on Monday, will travel to Cook Islands for the meeting later in the week.
A resolution at last year's meeting agreed on a united approach as the United States and China vie for influence, and mutual consultation before entering security arrangements.
Papua New Guinea formed a defence agreement with the United States in May. It has also sought to deepen economic ties with China, with Marape meeting Xi in Bejing last month where he attended the Belt and Road Forum.
New Zealand, yet to form a government after an election, will send a representative of the incoming National government and deputy prime minister from the caretaker Labour government.
Kirsty Needham - AAP