The Americans believe that the frozen billions are needed to secure a peace agreement between Ukraine and the invading power Russia, according to the diplomats, who spoke on condition of anonymity.
Earlier this week, the European Commission proposed that state assets be used as collateral for a loan of around 90 billion euros that will secure Ukraine's budget for the next two years.
Belgium, where most of the Russian money is held in a securities institution, is resisting for fear of huge Russian claims for damages in the future.
Bloomberg has contacted the US State Department, which has not responded.
