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."
The imposition of tariffs ups the pressure on May's minority Conservative government which depends on backing from the small Northern Irish Democratic Unionist Party (DUP) for their majority control of the British parliament.
Bombardier is the single largest manufacturing employer in Northern Ireland. The DUP's leader Arlene Foster said the ruling was not the end of the process. "There are further steps that will follow," she said in a statement.
Britain's business ministry said it would continue to work closely with the Canadian government to encourage all parties to reach a credible resolution as quickly as possible. It also criticised Boeing's role in the dispute.
"Boeing's position in this case is unjustified and frankly not what we would expect of a long-term partner to the UK - as well as damaging the wider global aerospace industry," a spokeswoman for the department said.