Headquartered in Samoa, SPREP has supported dozens of low-lying island states to raise awareness at UN climate conferences about the threat to their survival from rising sea levels caused by climate change.
The organisation employs more than 150 staff across Fiji, Solomon Islands, Vanuatu and the Republic of the Marshall Islands, and runs programs to reduce pollution, improve warning systems for severe weather and plan for disaster response to oil spills.
Sefanaia Nawadra, SPREP's director-general, said the US contributed funding and technical expertise, but other partners were expected to help it continue its work.
"There is a formal process that the US will need to follow to withdraw its membership of SPREP. They are a valued member of SPREP until that formal withdrawal process is completed," he said in a statement to Reuters.
"The impact of that will be determined as part of the details of the withdrawal process."
According to SPREP's annual report, its annual budget comes primarily from five donor countries, Australia, Britain, New Zealand, France and the US
China has also contributed $US200,000 ($A300,000) annually for several years.
A Pacific Islands government minister, who declined to be named because of the sensitivity of the issue, said the US move to withdraw from the organisation would negatively affect US influence in the region, where China is expanding ties.
Several Pacific Island nations face new hurdles to enter the United States.
Fiji, Vanuatu, Tuvalu and Tonga were listed on Wednesday among countries whose nationals must pay a costly visa bond to enter the US from January 21.
Tonga was listed in December as facing entry restrictions from January 1.
US Deputy Secretary of State Christopher Landau discussed migration in a call on Wednesday with Tonga Prime Minister Lord Fakafanua, he wrote on social media platform X.
The United States embassy in Suva referred requests for comment to the State Department, which did not immediately respond to a request for comment.
