Mr Rabuka is the Pacific's first leader to visit Washington DC since Mr Trump's election, taking meetings early in the new administration.
On Friday (AEDT), he met with Robert F. Kennedy, the new health secretary, wishing the controversial pick well in his new role.
Mr Kennedy's history in the blue continent is well known, with his anti-vaccine advocacy in Samoa linked to a 2019 measles outbreak which killed 83 people, many of them babies.
Mr Trump has moved quickly to assert control of the US government, including the bombshell dismantling of USAID, its independent agency which oversees foreign development assistance.
The freezing of US aid is likely to hit the Pacific hard, given widespread dependence on foreign assistance.
Funded projects include health assistance to fight disease, micro-financing loans crucial to business, and even overseas missions staffed by American diplomats.
BenarNews reported one senior health and HIV advisor working in Papua New Guinea as "devastated by the news of a funding pause on USAID programs in PNG".
In a social media post, Mr Rabuka said he was also concerned and took his message directly to the government.
"I hope that the situation will not hamper the plans that have been set in motion for the growth of USAID programs in the region, particularly in key areas such as oceans and climate resilience, health, security and digital connectivity," he said.
Mr Rabuka also met with the leader of the Congressional Pacific Caucus, Democratic lawmaker Ed Case of Hawaii.
"On climate change, I reiterated that this remains the single greatest threat to Fiji and the Pacific," Mr Rabuka said.
"While I recognise President Trump's decision to withdraw the US from the Paris Agreement, I expressed my hope that this will be reconsidered as the Pacific continues to bear the full brunt of the climate crisis."
The Pacific's peak environment agency, SPREP, is also "disheartened" by the US withdrawal from the Paris Agreement but noted it was not the first time Mr Trump had pulled out of the landmark climate treaty.
"The world has been in the position before where the US has stepped back from leading, or engaging in, climate discussions and this has not stopped climate action," SPREP Director General Sefanaia Nawadra said.
"It should not be any different if the US steps away this time. The transition to a renewable energy future is now well under way and the momentum for change is clear."
