The Marshall Islands nuclear tribunal says the United States may challenge the legality of the Marshall Islands bid for more US compensation.
The US carried out for 67 nuclear tests between 1946 and 1958.
Under previous agreements, the US paid money to the tribunal but the extent of the health problems related to the test has been bigger than expected.
This has prompted the Marshalls to seek further compensation but a spokesman for the tribunal, Bill Graham, says the US appears to be reluctant to consider this.
"It's my understanding that the request of the Marshall Islands to the Congress was basically referred back to the administration, with the idea in mind that they would re-examine the position that they had put forth to Congress in January this year saying that there was no legal basis for any additional compensation but I understand now that there is a legality study that may be underway."
Mr Graham says a study by the national cancer institute found a 9 percent increase in the number of cancer cases in the Marshall Islands due to radiation.