"One more meeting might be enough to finalize an arrangement. I can't be sure," said the U.S. official, who was familiar with the talks and spoke on the condition of anonymity, as reported by Reuters.
The two sides planned to meet again, possibly before the end of this year and most likely in Washington, the official told Reuters.
Scheduled commercial airline service has been suspended for decades as a result of Cold War animosity, but both sides quickly made resumption a priority upon detente last December.
Charter flights have long connected the United States and Cuba. Then Washington initiated new rules in January that more easily permit U.S. airlines to fly to Cuba.
However, U.S. and Cuban officials first need to negotiate a new arrangement before restarting scheduled service in which customers could book travel directly with airlines.
After that informal deal is reached, the two sides have agreed to work on updating a 1953 civil aviation agreement that is still valid but obsolete.
Officials from six U.S. government agencies met with counterparts from the Cuban Foreign Ministry and the Cuban Institute of Civil Aviation on Monday and Tuesday, the official said.
Safety and security cooperation was well advanced, largely because existing charter flights already meet U.S. Federal Aviation Administration (FAA) and Transportation Safety Administration (TSA) standards, the official said.
General U.S. tourism to Cuba is banned by the U.S. trade embargo of Cuba but certain Americans are allowed to go on specially sanctioned travel.
U.S. President Barack Obama has relaxed those restrictions, leading to a boom in U.S. citizen travel to Cuba, which is up more than 60 percent this year with 106,607 Americans arriving as of Sept. 20.
The market would grow further if the U.S. Congress were to lift either the tourism ban or the embargo.
"We don't have a deadline. We're eager. Our carriers are eager," the U.S. official said about reaching a deal. "Both sides see it as positive in and of itself but positive also as a signal of progress in the broader relationship."Major U.S. airlines including JetBlue Airways Corp, American Airlines Group Inc, Delta Air Lines Inc and United Air Lines have all expressed interest in scheduled service to Cuba.
(Reporting by Daniel Trotta; Editing by Lisa Shumaker)