The U.S. and Cuba have reached an understanding on restoring regularly scheduled commercial flights, Cuban and U.S. officials said Wednesday, a day before the anniversary of the two Cold War foes restoring diplomatic ties.

The diplomatic advance would help open the way for U.S. airlines to begin flying to Cuba within months in what would likely be the biggest step toward tighter economic ties since the two countries began normalizing relations last year. Officials on both sides said they had reached an understanding on key points and hoped to reach a formal deal within hours or days.

Teams who have been meeting since Monday in Washington "have made important advances in negotiating a memorandum of understanding on establishing regular flights between Cuba and the United States, and shortly they will be ready to announce a preliminary agreement on this issue," said Josefina Vidal, head of North American affairs for Cuba's foreign ministry.

State Department spokeswoman Kerry Humphrey said late Wednesday that the countries "are making progress but still negotiating."

Right now, American and Cuban travellers must fly on charter flights that are expensive and difficult to book, forcing travellers to buy paper tickets in Cuba or email documents and payment information back and forth with an agent in the U.S. Those flying often must arrive at the airport four hours in advance and pay high prices for baggage in excess of strict weight limits.

The U.S. Embassy in Havana reopened in July after five decades of Cold War tensions between the countries. (Carlos Garcia Rawlins/Reuters)

U.S. officials and aviation executives have speculated that Cuba could allow more than a dozen flights to and from the U.S. a day. It's unclear if those flights would completely replace charters, but they appear certain to create a surge in travel that would place heavy strain on Cuba's already overstrained tourist infrastructure. Hotels and private hostals are booked for months.

Authorized American travel to the island is up 50 per cent this year, Jeffrey DeLaurentis, who heads the U.S. Embassy in Havana, said Tuesday. While the majority of U.S. travellers are Cuban-Americans, there has been a sharp rise in Americans travelling for specially authorized purposes, particularly on educational tours known as people-to-people travel. Among Obama's regulatory changes this year was one permitting such travel to Cuba without specific Treasury Department permission.

Since the announcement last year that the U.S. would establish diplomatic relations and expand trade and tourism with Cuba, major U.S. airlines, including American, Delta, United, JetBlue and Southwest, have expressed interest in establishing regular flights from the U.S. to the island.

Cuba and the U.S. announced last week direct mail service would restart after a 52-year interruption. The governments had been speaking about restoring a postal link since President Barack Obama entered office, but those talks stalled when Cuba imprisoned U.S. contractor Alan Gross. He was freed in a prisoner exchange that sparked last year's declaration of detente.

On other issues, however, the U.S. and Cuba remain far apart. These include the billions of dollars in competing property claims, the status of fugitives in both countries, and Cuban protection of human rights.

The U.S. and Cuba re-opened embassies in each other's capitals this summer.

Thursday marks the one-year anniversary of the announcement by Presidents Obama and Raul Castro that they were ending a half-century of U.S.-Cuban enmity.