The US has said it has been "fair and open" in its handling of a trade row that threatens jobs at Bombardier in Belfast.
Wilbur Ross, the US Commerce Secretary, also said import tariffs on the C-Series jet could yet be lifted.
He said Washington understood "the political sensitivity" around the dispute in Canada and the UK.
However he added: "But the fundamentals remain: Even our best friends really have to play by the rules."
Mr Ross was speaking in London during a five-day visit and has discussed the issue with UK officials.
His commerce department has imposed preliminary import tariffs of 300% on the C-Series, after upholding a complaint brought by Boeing about unfair state aid.
The decision can still be reversed.
The row threatens jobs at Bombardier's Belfast plant, where parts of the C-Series jet, including the wings, are made.
Bombardier employs more than 4,000 people in Northern Ireland. A quarter of that workforce is involved in making C-Series wings.
The long-term future of the aircraft was given hope after Airbus took a 50.1% stake in the C-Series project.
The trade union Unite said Mr Ross' remarks "offered no assurance to Northern Ireland workers".
Regional co-ordinator Davy Thompson said: "If Wilbur Ross is genuine in his concern for Northern Ireland workers he should take the time to meet with their representatives in the course of the next five days.
"Boeing's case against Bombardier is entirely unmerited."