The US government has sought to intervene in Apple's appeal against an EU order to pay back up to €13 billion in taxes to Ireland, a source familiar with the matter has said.
Apple took its case to the Luxembourg-based General Court, Europe's second-highest, in December after the European Commission issued the record tax demand.
The Commission had said that the US company won sweetheart tax deals from the Irish government which amounted to illegal subsidies.
The decision was criticised by the Obama administration which said the European Union was helping itself to cash that should have ended up in the US.
The Trump administration has not said anything in public about the case so far.
"I can confirm the US filed an application with the European Union General Court to intervene in the case involving the retroactive application of state aid rules to Apple," the source said.
The source declined to be named because of the sensitivity of the matter.
The General Court is expected to hear the case in late 2018, another source with knowledge of the matter said.
Starbucks, Fiat Chrysler Automobiles and several other companies that were also ordered to pay back taxes to other EU countries have similarly challenged their EU rulings.