The Boeing announcement contrasted with other signs of growing hostility between the Trump administration and Iran. Last month, the United States imposed sanctions on 25 Iranian entities and individuals over what the White House called illegal Iranian missile tests. Mr. Trump included Iran on the blacklist of Muslim-majority countries subject to his proposed visa ban, which is currently blocked in the courts.

It was not clear from the Boeing announcement whether the tentative deal with Aseman Airlines had been negotiated under President Trump or under President Barack Obama. Boeing officials did not immediately return emailed requests for comment.

The announcement, posted on Boeing’s website, said Aseman intended to buy 30 Boeing 737 MAX aircraft valued at $3 billion, with rights to purchase an additional 30. Deliveries would be scheduled to start in 2022.

Aseman currently has a fleet of 31 aircraft, a mixture of Boeing, Airbus and Fokker models with an average age of 25 years, according to the website Planespotters.net.

The Boeing deal requires permission from the Office of Foreign Assets Control of the Treasury Department, which has regulatory authority over such transactions with Iran.

Not mentioned in the Boeing announcement, but still a significant hurdle for both sides, is how the company would be paid for the aircraft. Under sanctions that are still in place against Iran, the country cannot use the American banking system, so transactions must be conducted overseas in currencies other than the dollar.

Iran has one of the oldest commercial aircraft fleets in the world, a legacy of the prolonged estrangement with the United States that blocked the Iranians from refurbishing or replacing many of their American-built planes.