About 50 islanders from the nuclear test sites of Bikini, Enewetak, Rongelap and Utrik atolls protested outside the US mission in Majuro on Wednesday.
The atolls were the testing ground for the most destructive hydrogen bomb tests in the 1950s.
The tests left the Marshall Islands' environment devastated.
Since the 1960s America has funded compensation programmes for the islanders, but funding is due to run out on 30 September.
Demonstration leaders met briefly with Ambassador Greta Morris to discuss the impact on the islanders if funds are cut off.
To pay for the 67 nuclear weapons tested in the Marshall Islands from 1946 to 1958, the US government has shelled out 270 million dollars in compensation.
The Marshalls' government, however, filed a petition three years ago seeking a further two billion dollars in compensation, nuclear clean-up funding and health care aid.
Bush pressed to repay the United
State's debt to the Marshall
Wednesday's protest was sparked by anger over an amended US deal, which does not continue health and compensation programmes for the islands.
US officials said a review of the islanders' demands was nearly complete and would soon be passed to Congress.
But all these years of waiting can only compound the suffering of islanders, said protester Ichiro Mark, a resident of Bikini.
Mark has expressed grave concern at the prospects facing his people, some of whom are still gravely ill from the fallout of decades of nuclear testing.
"The US ruined Bikini," he said.
"They need to take on the burden they caused us. They can't just wash their hands clean from what they did to us."
In 1946 the USA began a nuclear weapons testing programme called Operation Crossroads on Bikini Atoll.
Chief Juda of Bikini agreed to evacuate the 167 islanders to Rongerik Atoll, east of Bikini, on the understanding they would be able to return once the tests were over.
In 1969 the US embarked on a long-term project to decontaminate land on Bikini Atoll.