May had urged U.S. President Donald Trump to help find a solution to the Boeing-Bombardier dispute which has put around 4,200 jobs at risk in the British province where the jet's wings are made.
Boeing accuses Bombardier of dumping its new CSeries passenger jet in the U.S. market and says the aircraft is being unfairly subsidized by Canada, a charge the Canadian firm denies.
The U.S. Commerce Department on Tuesday imposed a preliminary anti-subsidy 220 percent duty on the jets.
"Bitterly disappointed by initial Bombardier ruling," May said on Twitter.
"The government will continue to work with the company to protect vital jobs for Northern Ireland."