The US Trade Representative (USTR) office warned on Thursday that the British government's decision on the request for A350 launch aid would be pivotal in the dispute.
Earlier on Thursday, Airbus said it had lodged a request with the British government for reimbursable launch aid for its A350 programme.
"Our position has been clear: no new subsidies," USTR spokeswoman Neena Moorjani told AFP.
"The preferred outcome is a negotiated agreement on the elimination of subsides," she said.
"However, if additional subsidies are committed, we have said that we will resume litigation at the WTO and that remains our position."
The request, which also angered Airbus's US arch-rival Boeing, came as the US government and European Union battle to negotiate a deal on aircraft subsidies without going through the World Trade Organisation.